CheckLinkerFlag: Update documentation

- Added intro code block showing how to include this module.
- Added a list of variables that can affect the check. The
  CMAKE_REQUIRED_LINK_OPTIONS is skipped as it is currently internally
  overridden by the module's check command.
- Added examples section.
- Described command arguments as a list.
- Added included section describing the `LINKER:` (and `SHELL:`) prefix.
- Added link to the module in target_link_options() docs.