Start rewriting Flymake manual

Start rewriting Flymake manual

Missing the parts pertaining to the new customization API.

* 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.

* doc/misc/flymake.texi (Overview of Flymake): Rewrite a bit.
(Installing Flymake): Mostly scratch. Flymake comes with Emacs.
(Running the syntax check): Simplify.
(Viewing error messages): Dekete,
(Syntax check statuses): Rewrite.
(Troubleshooting): Simplify.
(Customizable variables): Rewrite.
(Extending Flymake): New chapter, empty for now.
(The legacy Proc backend): New chapter.
(Proc customizable variables)
(Adding support for a new syntax check tool)
(Implementation overview)
(Making a temporary copy)
(Locating a master file)
(Getting the include directories)
(Locating the buildfile)
(Starting the syntax check process)
(Parsing the output)
(Interaction with other modes)
(Example---Configuring a tool called via make)
(Example---Configuring a tool called directly): Rewrite a bit.