[doc] Remove Limitation that Compressed Block is Smaller than Uncompressed Content

This changes the size limit on compressed blocks to match those of the other
block types: they may not be larger than the `Block_Maximum_Decompressed_Size`,
which is the smaller of the `Window_Size` and 128 KB, removing the additional
restriction that had been placed on `Compressed_Block`s, that they be smaller
than the decompressed content they represent.

Several things motivate removing this restriction. On the one hand, this
restriction is not useful for decoders: the decoder must nonetheless be
prepared to accept compressed blocks that are the full
`Block_Maximum_Decompressed_Size`. And on the other, this bound is actually
artificially limiting. If block representations were entirely independent,
a compressed representation of a block that is larger than the contents of the
block would be ipso facto useless, and it would be strictly better to send it
as an `Raw_Block`. However, blocks are not entirely independent, and it can
make sense to pay the cost of encoding custom entropy tables in a block, even
if that pushes that block size over the size of the data it represents,
because those tables can be re-used by subsequent blocks.

Finally, as far as I can tell, this restriction in the spec is not currently
enforced in any Zstandard implementation, nor has it ever been. This change
should therefore be safe to make.

doc/zstd_compression_format.md

@@ -390,9 +390,7 @@ A block can contain any number of bytes (even zero), up to
 -  Window_Size
 -  128 KB
 
-A `Compressed_Block` has the extra restriction that `Block_Size` is always
-strictly less than the decompressed size.
-If this condition cannot be respected,
+If this condition cannot be respected when generating a `Compressed_Block`,
 the block must be sent uncompressed instead (`Raw_Block`).