bug#1297: font-lock-keywords-case-fold-search doc and usage

[Dan Nicolaescu]

font-lock-keywords-case-fold-search docstring should make it more clear
that font-lock-defaults erases it.

There is one victim in the tree.  `lisp-mode' does:

  (make-local-variable 'font-lock-keywords-case-fold-search)
  (setq font-lock-keywords-case-fold-search t)

but the value of font-lock-keywords-case-fold-search in a lisp-mode
buffer is nil, that's because font-lock-defaults does a
kill-local-variable on it...