Add legacy mode in documentation

lib/legacy/zstd_v01.c

@@ -2001,6 +2001,8 @@ size_t ZSTDv01_decompress(void* dst, size_t maxDstSize, const void* src, size_t
     return ZSTDv01_decompressDCtx(&ctx, dst, maxDstSize, src, srcSize);
 }
 
+/* ZSTD_errorFrameSizeInfoLegacy() :
+   assumes `cSize` and `dBound` are _not_ NULL */
 static void ZSTD_errorFrameSizeInfoLegacy(size_t* cSize, unsigned long long* dBound, size_t ret)
 {
     *cSize = ret;