Help: Clarify permission-related command options

The previous docs did not make clear that at most only one of the three
permissions-related options can be given for configure_file() or
file(GENERATE) and that USE_SOURCE_PERMISSIONS is already the
default behavior for these commands.

Use consistent wording to refer to the input and output files for
configure_file().

Add missing "versionadded" directives on some of these keywords.

Use the consistent wording and keyword ordering between the
configure_file(), file(GENERATE) and file(COPY) commands.