update man

programs/zstd.1.md

@@ -131,6 +131,7 @@ the last one takes effect.
     In all cases, the nb of threads is capped to `ZSTDMT_NBWORKERS_MAX`,
     which is either 64 in 32-bit mode, or 256 for 64-bit environments.
     This modifier does nothing if `zstd` is compiled without multithread support.
+    Note that memory usage increases with each thread.
 * `--single-thread`:
     Use a single thread for both I/O and compression.
     As compression is serialized with I/O, this can be slightly slower.
@@ -157,7 +158,7 @@ the last one takes effect.
     when combined with multiple worker threads (>=2).
 * `--long[=#]`:
     enables long distance matching with `#` `windowLog`, if `#` is not
-    present it defaults to `27`.
+    present it defaults to `27`. The highest possible value is 31
     This increases the window size (`windowLog`) and memory usage for both the
     compressor and decompressor.
     This setting is designed to improve the compression ratio for files with

programs/zstdcli.c

@@ -1540,7 +1540,12 @@ int main(int argCount, const char* argv[])
     /* check compression level limits */
     {   int const maxCLevel = ultra ? ZSTD_maxCLevel() : ZSTDCLI_CLEVEL_MAX;
         if (cLevel > maxCLevel) {
-            DISPLAYLEVEL(2, "Warning : compression level higher than max, reduced to %i \n", maxCLevel);
+            DISPLAYLEVEL(2, "Warning : compression level higher than max, reduced to %i. ", maxCLevel);
+            DISPLAYLEVEL(2, "Specify --ultra to raise the limit to 22 and use "
+                            "--long=31 for maximum compression. Note that this "
+                            "requires high amounts of memory, and the resulting data "
+                            "might be rejected by third-party decoders and is "
+                            "therefore only recommended for archival purposes. \n");            
             cLevel = maxCLevel;
     }   }
 #endif