New property dynamic-docstring-function for docstrings.

New property dynamic-docstring-function for docstrings.
* src/doc.c (Fdocumentation): Handle new property
dynamic-docstring-function to replace the old ad-advice-info.
* lisp/emacs-lisp/advice.el: Use new dynamic docstrings.
(ad-make-advised-definition-docstring, ad-advised-definition-p):
Use dynamic-docstring-function instead of ad-advice-info.
(ad--make-advised-docstring): New function extracted from
ad-make-advised-docstring.
(ad-make-advised-docstring): Use it.
* lisp/progmodes/sql.el (sql--make-help-docstring): New function, extracted
from sql-help.
(sql-help): Use it with dynamic-docstring-function.