Cleaning up Check<Lang>SourceCompiles module docs

The CMake 3.19 release includes a new CheckSourceCompiles module which replaces the individual Check<Lang>SourceCompiles modules. Indeed, those modules now simply forward to the new one. The old modules still have all the documentation though and this is likely to get out of date over time. Should we strip down the old docs to simply say the module is deprecated and that its associated command now just forwards to the new module’s one?

Cc: @robert.maynard

The CheckLinkerFlag module also still refers to the old Check<Lang>SourceCompiles modules instead of the new CheckSourceCompiles module. I will update its docs to refer to the new module instead (I’m making a pass through all the docs new for the 3.19 release and expect to put up a merge request for those changes tomorrow).

And looks like we have the same situation for the new CheckCompilerFlag module replacing the existing Check<Lang>CompilerFlag modules too. And CheckSourceRuns replacing Check<Lang>SourceRuns. So same question, should we strip down the old module docs for those too?