Deprecate --cache.* ?

Currently, most of the remaining references to "cache" in Documantation/
are related to the --cache.* flag of four git commands.

git-dif-*index* uses --cached to mean "do not consider on-disk files at all"

git-fsck-objects uses --cache to consider index entries as head nodes for 
unreachability traces.

git-ls-files --cached shows all files recorded in the index.

git-update-*index* --cacheinfo is used to place a "fake" entry in the index

git-update-index also has the option --index-info which does something different.

My suggestion is that these cache-references be deprecated in favor of
--index, --in-index, --index-only, --insert-index, or something along those lines.

Backward compability could be ensured by having both versions of the flags
around for a while and issuing a warning when the old form is used.

Good idea? Bad? Stupid? Do we want to keep "cache" around?