docs: expand regex syntax description in usage text for search, `…

…searchset` & `replace`

[skip ci]

src/cmd/replace.rs

@@ -18,7 +18,10 @@ Usage:
     qsv replace --help
 
 replace arguments:
-    <pattern>              Regular expression to match.
+    <pattern>              Regular expression pattern to match. Uses Rust regex syntax.
+                           See https://docs.rs/regex/latest/regex/index.html#syntax
+                           or https://regex101.com with the Rust flavor for more info.
+    <input>                The CSV file to read. If not given, reads from stdin.
     <replacement>          Replacement string. Set to '<NULL>' if you want to
                            replace matches with ''.
 replace options:

src/cmd/search.rs

@@ -19,6 +19,12 @@ Usage:
     qsv search [options] <regex> [<input>]
     qsv search --help
 
+search arguments:
+    <regex>                Regular expression to match. Uses Rust regex syntax.
+                           See https://docs.rs/regex/latest/regex/index.html#syntax
+                           or https://regex101.com with the Rust flavor for more info.
+    <input>                The CSV file to read. If not given, reads from stdin.
+
 search options:
     -i, --ignore-case      Case insensitive search. This is equivalent to
                            prefixing the regex with '(?i)'.

src/cmd/searchset.rs

@@ -25,7 +25,13 @@ Usage:
     qsv searchset [options] (<regexset-file>) [<input>]
     qsv searchset --help
 
-search options:
+searchset arguments:
+    <regex>                    Regular expression to match. Uses Rust regex syntax.
+                               See https://docs.rs/regex/latest/regex/index.html#syntax
+                               or https://regex101.com with the Rust flavor for more info.
+    <input>                    The CSV file to read. If not given, reads from stdin.
+
+searchset options:
     -i, --ignore-case          Case insensitive search. This is equivalent to
                                prefixing the regex with '(?i)'.
     --literal                  Treat the regex as a literal string. This allows