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?