Add full documentation on new Flymake API

Add full documentation on new Flymake API

Also, as a minor addition to this API, set flymake-text
on the diagnostic overlay.  This enables a good example in
the section "Customization Flymake annotations".

* doc/misc/flymake.texi (Overview of Flymake)
(Syntax check statuses): Rework.
(Backend exceptions): Rename from "Troubleshooting"
(Customizable variables): Add flymake-start-on-flymake-mode. Rework.
(Extending Flymake): Write chapter.
(Customizing Flymake annotations, Flymake backends)
(Flymake utility functions, An annotated example backend):
New sections and subsections

* lisp/progmodes/flymake.el (flymake-diagnostic-functions)
(flymake-diagnostic-types-alist): Rework docstring.
(flymake--highlight-line): Set and use flymake-text property in overlay.
(flymake-goto-next-error, flymake-goto-prev-error): Fix funny quotes.