Use ZSTD Error codes, improve explanation of ZSTD_loadCEntropy() and ZSTD_loadDEntropy()

lib/decompress/zstd_decompress_internal.h

@@ -160,7 +160,7 @@ struct ZSTD_DCtx_s
 
 /*! ZSTD_loadDEntropy() :
  *  dict : must point at beginning of a valid zstd dictionary.
- * @return : size of entropy tables read */
+ * @return : size of dictionary header (size of magic number + dict ID + entropy tables) */
 size_t ZSTD_loadDEntropy(ZSTD_entropyDTables_t* entropy,
                    const void* const dict, size_t const dictSize);