Simplify Flymake user documentation

Simplify Flymake user documentation

Fix some confusing parts of Flymake user documentation, like a
repeated listing of situations leading to a syntax check.
Documentation is now divided into three chapters only: "Using
Flymake", "Extending Flymake", and a chapter for the legacy backend.

Obsolete mention of "errors" and "lines" is replaced by
"diagnostics/problems" and "regions" where appropriate.

* doc/misc/flymake.texi (Overview of Flymake): Delete chapter.
(Using Flymake): Merge with chapter "Overview of Flymake".
(Installing Flymake): Delete chapter.
(Flymake mode, Running the syntax check)
(Navigating to error lines): Delete sections.
(Syntax check statuses): Review and augment with x-references.
(Backend exceptions): Mention shortcut in menu.

* lisp/progmodes/flymake.el (flymake-menu): Tweak minor mode
menu to not speak of "errors".
(flymake-goto-next-error, flymake-goto-prev-error): Fix
wording.