policy change : ZSTDMT automatically caps nbThreads to ZSTDMT_NBTHREADS_MAX (#760)

Previously, ZSTDMT would refuse to create the compressor.
Also : increased ZSTDMT_NBTHREADS_MAX to 256,
updated doc,
and added relevant test

programs/zstd.1.md

@@ -108,6 +108,7 @@ the last one takes effect.
 * `-T#`, `--threads=#`:
     Compress using `#` threads (default: 1).
     If `#` is 0, attempt to detect and use the number of physical CPU cores.
+    In all cases, the nb of threads is capped to ZSTDMT_NBTHREADS_MAX==256.
     This modifier does nothing if `zstd` is compiled without multithread support.
 * `-D file`:
     use `file` as Dictionary to compress or decompress FILE(s)