CPackRPM: Fix misleading CPACK_RPM_SPEC_INSTALL_POST docs
The docs for CPACK_RPM_SPEC_INSTALL_POST have long indicated that this variable is deprecated, but the alternative variable the docs recommend is for unrelated functionality. This MR corrects that detail and also includes some very minor cleanup of formatting in the same file in a separate commit.
This is a docs-only change, no functionality is affected.