Added comments on I/O buffer sizes for streaming

It seems this is still a confusing topic,
as in https://github.com/klauspost/compress/issues/109 .

doc/zstd_manual.html

@@ -18,7 +18,7 @@
 <li><a href="#Chapter8">Advanced decompression API</a></li>
 <li><a href="#Chapter9">Streaming</a></li>
 <li><a href="#Chapter10">Streaming compression - HowTo</a></li>
-<li><a href="#Chapter11">This is a legacy streaming API, and can be replaced by ZSTD_CCtx_reset() and</a></li>
+<li><a href="#Chapter11">This following is a legacy streaming API.</a></li>
 <li><a href="#Chapter12">Equivalent to:</a></li>
 <li><a href="#Chapter13">Alternative for ZSTD_compressStream2(zcs, output, input, ZSTD_e_continue).</a></li>
 <li><a href="#Chapter14">Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_flush).</a></li>
@@ -592,9 +592,10 @@ size_t ZSTD_freeCStream(ZSTD_CStream* zcs);
 
 <pre><b>size_t ZSTD_CStreamInSize(void);    </b>/**< recommended size for input buffer */<b>
 </b></pre><BR>
-<pre><b>size_t ZSTD_CStreamOutSize(void);   </b>/**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block in all circumstances. */<b>
+<pre><b>size_t ZSTD_CStreamOutSize(void);   </b>/**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block. */<b>
 </b></pre><BR>
-<a name="Chapter11"></a><h2>This is a legacy streaming API, and can be replaced by ZSTD_CCtx_reset() and</h2><pre> ZSTD_compressStream2(). It is redundant, but is still fully supported.
+<a name="Chapter11"></a><h2>This following is a legacy streaming API.</h2><pre> It can be replaced by ZSTD_CCtx_reset() and ZSTD_compressStream2().
+ It is redundant, but remains fully supported.
  Advanced parameters and dictionary compression can only be used through the
  new API.
 <BR></pre>
@@ -608,7 +609,7 @@ size_t ZSTD_freeCStream(ZSTD_CStream* zcs);
 
 <a name="Chapter13"></a><h2>Alternative for ZSTD_compressStream2(zcs, output, input, ZSTD_e_continue).</h2><pre> NOTE: The return value is different. ZSTD_compressStream() returns a hint for
  the next read size (if non-zero and not an error). ZSTD_compressStream2()
- returns the number of bytes left to flush (if non-zero and not an error).
+ returns the minimum nb of bytes left to flush (if non-zero and not an error).
  
 <BR></pre>
 

lib/zstd.h

@@ -657,12 +657,28 @@ ZSTDLIB_API size_t ZSTD_compressStream2( ZSTD_CCtx* cctx,
                                          ZSTD_inBuffer* input,
                                          ZSTD_EndDirective endOp);
 
+
+/* These buffer sizes are softly recommended.
+ * They are not required : ZSTD_compressStream*() happily accepts any buffer size, for both input and output.
+ * Respecting the recommended size just makes it a bit easier for ZSTD_compressStream*(),
+ * reducing the amount of memory shuffling and buffering, resulting in minor performance savings.
+ *
+ * However, note that these recommendations are from the perspective of a C caller program.
+ * If the streaming interface is invoked from some other language,
+ * especially managed ones such as Java or Go, through a foreign function interface such as jni or cgo,
+ * a major performance rule is to reduce crossing such interface to an absolute minimum.
+ * It's not rare that performance ends being spent more into the interface, rather than compression itself.
+ * In which cases, prefer using large buffers, as large as practical,
+ * for both input and output, to reduce the nb of roundtrips.
+ */
 ZSTDLIB_API size_t ZSTD_CStreamInSize(void);    /**< recommended size for input buffer */
-ZSTDLIB_API size_t ZSTD_CStreamOutSize(void);   /**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block in all circumstances. */
+ZSTDLIB_API size_t ZSTD_CStreamOutSize(void);   /**< recommended size for output buffer. Guarantee to successfully flush at least one complete compressed block. */
+
 
 /*******************************************************************************
- * This is a legacy streaming API, and can be replaced by ZSTD_CCtx_reset() and
+ * This following is a legacy streaming API.
- * ZSTD_compressStream2(). It is redundant, but is still fully supported.
+ * It can be replaced by ZSTD_CCtx_reset() and ZSTD_compressStream2().
+ * It is redundant, but remains fully supported.
  * Advanced parameters and dictionary compression can only be used through the
  * new API.
  ******************************************************************************/
@@ -679,7 +695,7 @@ ZSTDLIB_API size_t ZSTD_initCStream(ZSTD_CStream* zcs, int compressionLevel);
  * Alternative for ZSTD_compressStream2(zcs, output, input, ZSTD_e_continue).
  * NOTE: The return value is different. ZSTD_compressStream() returns a hint for
  * the next read size (if non-zero and not an error). ZSTD_compressStream2()
- * returns the number of bytes left to flush (if non-zero and not an error).
+ * returns the minimum nb of bytes left to flush (if non-zero and not an error).
  */
 ZSTDLIB_API size_t ZSTD_compressStream(ZSTD_CStream* zcs, ZSTD_outBuffer* output, ZSTD_inBuffer* input);
 /** Equivalent to ZSTD_compressStream2(zcs, output, &emptyInput, ZSTD_e_flush). */