bug#16993: advice-add hides docstring

[Stefan]

> Some advice-add'ed functions lose their docstring.  Current examples
> from "emacs -Q" include `insert-directory', `rename-buffer' and
> `create-file-buffer'.

That's an incompatibility with Snarf-documentation: Snarf-documentation
scans the DOC file, finds the doc of rename-buffer, and tries to stick
the corresponding DOC offset in rename-buffer's "docstring slot".
This should set the "docstring slot" of the #<subr rename-buffer>, but
instead (symbol-function 'rename-buffer) returns an `advice--p' object,
so Snarf-documentation tries to set the docstring slot of that object
(which fails because the vector doesn't have a slot for it, but even if
it succeeded it wouldn't be what we want).

We can either fix it by teaching Snarf-documentation about advised
function (yuck!) or by not advising functions that are documented in
DOC.  IOW the right fix is to get rid of those advices in uniquify.el.


        Stefan