- Added intro code block showing how to include this module.
- Reworded module introduction.
- Used "command" instead of "function".
- Reworded command arguments and descriptions.
- Added the "See Also" section.
As a consequence, `GNUInstallDirs_get_absolute_install_dir` does not
alter the `var` argument (the relative path argument).
Closes: #26834
Signed-off-by: Cristian Le <git@lecris.dev>