Prefixed manual describe-function and api overview

[Philippe Vaucher <philippe.vaucher@gmail.com>]

Hello,

Following up on our previous discussions, here is a working
implementation of the completion mechanism based on the manual:

https://github.com/Silex/prefixed-manual.el/blob/master/prefixed-manual.el

At the moment it uses https://melpa.org/#/dash and
https://melpa.org/#/s so make sure you have these dependencies. Then
to try it out, test with `M-x prefixed-manual-describe-function` and
`M-x prefixed-manual-overview`. The later still needs a lot of work
but it gives an idea.

Here are my questions/plans:
- If there is interest for some parts for the core, I'll rewrite this
in vanilla Emacs.
- I'm not sure I like "prefixed-manual", given this is more about
"manual-helpers". Feel free to suggest a better name.
- In `prefixed-manual-overview`, I'd like to display the full
signature of functions. For example display "(substring-no-properties
STRING &optional FROM TO)" instead of "substring-no-properties". If
you know a simple way I'm interested. I tried to use something like
`(get 'substring-no-properties 'function-documentation)` or play with
`documentation-property` but couldn't find easily and gave up.

Kind regards,
Philippe