Overhaul annotation-function to match affixation-function

Overhaul annotation-function to match affixation-function

* doc/lispref/minibuf.texi (Programmed Completion): Rework
annotation-function and affixation-function.

* lisp/help-fns.el (help--symbol-completion-table-annotation): Rename
from help--symbol-completion-table-affixation.
(help--symbol-completion-table): Use
help--symbol-completion-table-annotation.

* lisp/minibuffer.el (minibuffer-completion-help): Interpret
annotation-function with more sophistication.

* lisp/simple.el (read-extended-command): Use
read-extended-command--annotation
(read-extended-command--annotation): Rename from
read-extended-command--affixation