Merge #13340: doc: remove leftover check-doc documentation

93843f6891 doc: remove leftover check-doc documentation (fanquake)

Pull request description:

  Remove leftover check-doc.py documentation. Mentioned [here](https://github.com/bitcoin/bitcoin/pull/13281#issuecomment-392010168), it's now [here](https://github.com/bitcoin/bitcoin/tree/master/test/lint#check-docpy).

Tree-SHA512: 95a0ac221ffae109c1d4baf18a9220cf993fc07c005920a0bd09abdf52e8fb298e3b5df31fa18887719c5080d8531d18b84b7bd9c7c664ee2501ccd9e0975eb6

contrib/devtools/README.md

@@ -2,12 +2,6 @@ Contents
 ========
 This directory contains tools for developers working on this repository.
 
-check-doc.py
-============
-
-Check if all command line args are documented. The return value indicates the
-number of undocumented args.
-
 clang-format-diff.py
 ===================