Docstring and manual updates for completion-at-point-functions.

Docstring and manual updates for completion-at-point-functions.

* doc/lispref/minibuf.texi (Basic Completion): Define "completion table".
Move completion-in-region to Completion in Buffers node.
(Completion Commands): Use "completion table" terminology.
(Completion in Buffers): New node.

* doc/lispref/modes.texi (Hooks): add-hook can be used for abnormal hooks too.
(Setting Hooks): Update minor mode usage example.
(Major Mode Conventions): Note that completion-at-point-functions
should be altered locally.  Add xref to Completion in Buffers.

* lisp/minibuffer.el (completion-at-point-functions): Doc fix.