Add Automatic Variables section.

Author: aaronriekenberg

scripts/generate_manual.sh

@@ -11,6 +11,7 @@ echo "## Manual for rust-parallel $VERSION"
 echo '
 1. [Command line options](#command-line-options)
 1. [Commands from arguments](#commands-from-arguments)
+   1. [Automatic variables](#automatic-variables)
 1. [Commands from stdin](#commands-from-stdin)
 1. [Command and initial arguments on command line](#command-and-initial-arguments-on-command-line)
 1. [Reading multiple inputs](#reading-multiple-inputs)
@@ -58,14 +59,18 @@ echo '
 $ rust-parallel gzip -k ::: *.html
 ```'
 
-echo 'Variables `{0}`, `{1}`, etc are automatically available based on the number of arguments.  `{0}` will be replaced by the entire input line, and other groups match individual argument groups  This is useful for building more complex command lines.  For example:
+echo '### Automatic Variables'
+
+echo 'When using commands from arguments, numbered variables `{0}`, `{1}`, etc are automatically available based on the number of arguments.  `{0}` will be replaced by the entire input line, and other groups match individual argument groups  This is useful for building more complex command lines.  For example:
 '
 
 echo '```
 $ rust-parallel echo group0={0} group1={1} group2={2} group3={3} group2again={2} ::: A B ::: C D ::: E F G'
 $RUST_PARALLEL echo group0={0} group1={1} group2={2} group3={3} group2again={2} ::: A B ::: C D ::: E F G
 echo '```'
 
+echo 'Internally these variables are implemented using an auto-generated [regular expression](#regular-expression).  If a regular expression is manually specified this will override the auto-generated one.'
+
 echo '## Commands from stdin
 
 Run complete commands from stdin.