Improve documentation on tweaking <Package>_ENABLE_<YYY> and HAVE_<PACKAGE>_<YYY> vars (trilinos/Trilinos#11612)

[bartlettroscoe]

Description

This explains the changes needed in:

• PR trilinos/Trilinos#11612.

Instructions for Reviewers

The updated sections can be viewed as rendered by GitHub at:

• ${PACKAGE_NAME}_ENABLE_${OPTIONAL_DEP_PACKAGE_NAME}

• HAVE_<PACKAGE_NAME_UC>_<OPTIONAL_DEP_PACKAGE_NAME_UC>

• TriBITS auto-enables/disables done using non-cache local variables

• How to check for and tweak TriBITS "ENABLE" cache variables

• How to tweak downstream TriBITS "ENABLE" variables during package configuration

[bartlettroscoe]

@cgcgcg, could you please take a look at the updated documentation in this PR related to https://github.com/trilinos/Trilinos/pull/11612? NOTE: There are links to the rendered sections above to make it easy to read over the updated documention.

[bartlettroscoe]

@KyleFromKitware, can you please look over the updated documentation?

[cgcgcg]

https://github.com/bartlettroscoe/TriBITS/blob/72ff46705c8093c7c177e41e59221f6a54129d21/tribits/doc/guides/TribitsGuidesBody.rst?plain=1#L1812 were -> where