Start rewriting Flymake manual

Start rewriting Flymake manual

* doc/misc/flymake.texi (Overview of Flymake): Rewrite a bit.
(Installing Flymake): Delete most of this.
(Running the syntax check): Mention flymake-start.
(Navigating to error lines): Rewrite.
(Viewing error messages): Commente out.
(Syntax check statuses, Troubleshooting): Rewrite a bit.
(Customizable variables): New section under "Using
Flymake".  Don't mention any proc variables here.
(Configuring Flymake): Delete
(Proc backend): New chapter
(Proc customization variables): New chapter.