added mode documentation on ZSTD_CLEVEL

providing range restriction [1-19] explicitly
partially answering #1829

programs/README.md

@@ -173,10 +173,13 @@ Benchmark arguments :
 ```
 
 #### Restricted usage of Environment Variables
-Using environment variables to set compression/decompression parameters has security implications. Therefore,
+Using environment variables to set parameters has security implications.
-we intentionally restrict its usage. Currently, only `ZSTD_CLEVEL` is supported for setting compression level.
+Therefore, this avenue is intentionally restricted.
+Only `ZSTD_CLEVEL` is supported currently, for setting compression level.
+`ZSTD_CLEVEL` can be used to set the level between 1 and 19 (the "normal" range).
 If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message.
-Note that command line options will override corresponding environment variable settings.
+`ZSTD_CLEVEL` just replaces the default compression level (`3`).
+It can be overridden by corresponding command line arguments.
 
 #### Long distance matching mode
 The long distance matching mode, enabled with `--long`, is designed to improve

programs/zstd.1

@@ -1,5 +1,5 @@
 .
-.TH "ZSTD" "1" "September 2019" "zstd 1.4.4" "User Commands"
+.TH "ZSTD" "1" "October 2019" "zstd 1.4.4" "User Commands"
 .
 .SH "NAME"
 \fBzstd\fR \- zstd, zstdmt, unzstd, zstdcat \- Compress or decompress \.zst files
@@ -206,6 +206,9 @@ add integrity check computed from uncompressed data (default: enabled)
 \fB\-\-\fR
 All arguments after \fB\-\-\fR are treated as files
 .
+.SS "Restricted usage of Environment Variables"
+Using environment variables to set parameters has security implications\. Therefore, this avenue is intentionally restricted\. Only \fBZSTD_CLEVEL\fR is supported currently, for setting compression level\. \fBZSTD_CLEVEL\fR can be used to set the level between 1 and 19 (the "normal" range)\. If the value of \fBZSTD_CLEVEL\fR is not a valid integer, it will be ignored with a warning message\. \fBZSTD_CLEVEL\fR just replaces the default compression level (\fB3\fR)\. It can be overridden by corresponding command line arguments\.
+.
 .SH "DICTIONARY BUILDER"
 \fBzstd\fR offers \fIdictionary\fR compression, which greatly improves efficiency on small files and messages\. It\'s possible to train \fBzstd\fR with a set of samples, the result of which is saved into a file called a \fBdictionary\fR\. Then during compression and decompression, reference the same dictionary, using command \fB\-D dictionaryFileName\fR\. Compression of small files similar to the sample set will be greatly improved\.
 .

programs/zstd.1.md

@@ -214,6 +214,16 @@ the last one takes effect.
 * `--`:
     All arguments after `--` are treated as files
 
+### Restricted usage of Environment Variables
+
+Using environment variables to set parameters has security implications.
+Therefore, this avenue is intentionally restricted.
+Only `ZSTD_CLEVEL` is supported currently, for setting compression level.
+`ZSTD_CLEVEL` can be used to set the level between 1 and 19 (the "normal" range).
+If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message.
+`ZSTD_CLEVEL` just replaces the default compression level (`3`).
+It can be overridden by corresponding command line arguments.
+
 
 DICTIONARY BUILDER
 ------------------

programs/zstdgrep.1

@@ -1,5 +1,5 @@
 .
-.TH "ZSTDGREP" "1" "September 2019" "zstd 1.4.4" "User Commands"
+.TH "ZSTDGREP" "1" "October 2019" "zstd 1.4.4" "User Commands"
 .
 .SH "NAME"
 \fBzstdgrep\fR \- print lines matching a pattern in zstandard\-compressed files

programs/zstdless.1

@@ -1,5 +1,5 @@
 .
-.TH "ZSTDLESS" "1" "September 2019" "zstd 1.4.4" "User Commands"
+.TH "ZSTDLESS" "1" "October 2019" "zstd 1.4.4" "User Commands"
 .
 .SH "NAME"
 \fBzstdless\fR \- view zstandard\-compressed files