Add legacy mode in documentation

lib/legacy/zstd_v02.c

@@ -3314,7 +3314,9 @@ static size_t ZSTD_decompress(void* dst, size_t maxDstSize, const void* src, siz
     return ZSTD_decompressDCtx(&ctx, dst, maxDstSize, src, srcSize);
 }
 
-MEM_STATIC void ZSTD_errorFrameSizeInfoLegacy(size_t* cSize, unsigned long long* dBound, size_t ret)
+/* 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;
     *dBound = ZSTD_CONTENTSIZE_ERROR;