Minor grep.el doc string clarifications

Minor grep.el doc string clarifications

* lisp/progmodes/grep.el (grep-find-ignored-directories)
(grep-compute-defaults, grep, grep-expand-template)
(grep-highlight-matches, grep-use-null-device)
(grep-find-command, grep-find-template, grep-expand-template)
(rgrep): Minor doc string clarifications.