Add legacy mode in documentation

lib/legacy/zstd_v01.h

@@ -42,6 +42,8 @@ size_t ZSTDv01_decompress( void* dst, size_t maxOriginalSize,
                                 or an errorCode if it fails (which can be tested using ZSTDv01_isError())
      dBound (output paramter) : an upper-bound for the decompressed size of the data in the frame
                                 or ZSTD_CONTENTSIZE_ERROR if an error occurs
+
+     note : assumes `cSize` and `dBound` are _not_ NULL.
  */
 void ZSTDv01_findFrameSizeInfoLegacy(const void *src, size_t srcSize,
                                      size_t* cSize, unsigned long long* dBound);