Fix incorrect sly-autodoc inhibition in sly-mrepl

* contrib/sly-autodoc.el (sly-autodoc-inhibit-autodoc): New variable.
(sly-autodoc): Inhibit according to
`sly-autoloads-inhibit-autodoc' variable.

* contrib/sly-mrepl.el (sly-mrepl-mode): Set
`sly-autodoc-inhibit-autodoc' to
`sly-mrepl-inside-string-or-comment-p'
(sly-mrepl-inside-string-or-comment-p): New function.