clarifications on the meaning of field `Block_Size`

following comments from Intel's Smita Kumar.

doc/zstd_compression_format.md

@@ -16,7 +16,7 @@ Distribution of this document is unlimited.
 
 ### Version
 
-0.3.2 (17/07/19)
+0.3.3 (16/08/19)
 
 
 Introduction
@@ -358,6 +358,7 @@ It may be followed by an optional `Content_Checksum`
 __`Block_Type`__
 
 The next 2 bits represent the `Block_Type`.
+`Block_Type` influences the meaning of `Block_Size`.
 There are 4 block types :
 
 |    Value     |      0      |      1      |         2          |     3     |
@@ -384,9 +385,12 @@ There are 4 block types :
 __`Block_Size`__
 
 The upper 21 bits of `Block_Header` represent the `Block_Size`.
-`Block_Size` is the size of the block excluding the header.
-A block can contain any number of bytes (even zero), up to
-`Block_Maximum_Decompressed_Size`, which is the smallest of:
+When `Block_Type` is `Compressed_Block` or `Raw_Block`,
+`Block_Size` is the size of `Block_Content`, hence excluding `Block_Header`.  
+When `Block_Type` is `RLE_Block`, `Block_Content`’s size is always 1,
+and `Block_Size` represents the nb of times this byte must be repeated.
+A block can contain and decompress into any number of bytes (even zero),
+up to `Block_Maximum_Decompressed_Size`, which is the smallest of:
 -  Window_Size
 -  128 KB
 
@@ -1653,6 +1657,7 @@ or at least provide a meaningful error code explaining for which reason it canno
 
 Version changes
 ---------------
+- 0.3.3 : clarifications for field Block_Size
 - 0.3.2 : remove additional block size restriction on compressed blocks
 - 0.3.1 : minor clarification regarding offset history update rules
 - 0.3.0 : minor edits to match RFC8478