Corrections and tweaks to Help chapter in Emacs manual.

Corrections and tweaks to Help chapter in Emacs manual.

* doc/emacs/help.texi (Help, Help Summary): Eliminate the unnecessary "help
option" terminology.
(Key Help): Add command names.  Define "documentation string".
(Name Help): Remove an over-long joke.
(Apropos): Document prefix args.  Remove duplicated descriptions.
(Help Mode): Add C-c C-b to table.  Update TAB binding.
(Package Keywords): Rename from "Library by Keyword".  Describe
new package menu interface.
(Help Files, Help Echo): Tweak description.

* doc/emacs/mini.texi: Various copyedits.