should doc on case-fold-search mention that there are other such vars also?

[Drew Adams]

What about mentioning, in the Emacs manual section on `case-fold-search',
that there are also other, similar options that control case sensitivity in
different contexts. We could suggest, for instance, that the user do
`apropos-variable' on "case-fold-search" to find them all.

A recent question on help-gnu-emacs brought this to mind. The user was
unaware of `tags-case-fold-search', and couldn't figure out how to use
`case-fold-search' to make tags search case-sensitive.