CMake: various small non-functional improvements

CMake: various small non-functional improvements

These are based on review comments for the 2.6 backport.
But since they apply to the original master implementation
as well, I address them in this separate patch.

- Add documentation to contrib/cmake/*.py
- Fix grammar in README.cmake.md
- Update a TODO in CMakeLists.txt to better reflect
  the status quo
- Fix indentation in unit_tests' Makefile.am

Change-Id: I4e16767ee221e1aefdd18d13b3411c27d8dd844a
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/342
Message-Id: <20230919155635.708557-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27043.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>