docs/string: Add paragraph on comparison with unix tools

[ci skip]

sphinx_doc_src/cmds/string.rst

@@ -226,6 +226,21 @@ And some other things:
 - ``^`` is the start of the string or line, ``$`` the end.
 - ``|`` is "alternation", i.e. the "or".
 
+Comparison to other tools
+-------------------------
+
+Most operations ``string`` supports can also be done by external tools. Some of these include ``grep``, ``sed`` and ``cut``.
+
+If you are familiar with these, it is useful to know how ``string`` differs from them.
+
+In contrast to these classics, ``string`` reads input either from stdin or as arguments. ``string`` also does not deal with files, so it requires redirections to be used with them.
+
+In contrast to ``grep``, ``string``\ s `match` and `replace` default to glob-mode. If set to regex-mode, they use PCRE regular expressions, which is comparable to ``grep``\ s `-P` option. `match` defaults to printing just the match, which is like ``grep`` with `-o` (use `--entire` to enable grep-like behavior).
+
+Like ``sed``\ s `s/` command, ``string replace`` still prints strings that don't match. ``sed``\ s `-n` in combination with a `/p` modifier or command is like ``string replace -f``.
+
+``string split somedelimiter`` is a replacement for ``tr somedeliminter \n`.
+
 Examples
 --------