Improve the Flymake manual

Improve the Flymake manual

* doc/misc/flymake.texi: Add a 'coding' cookie.  Add a
@syncodeindex directive for @vindex.  Use 2 spaces between
sentences.  Lower-case @cindex entries.
(Overview of Flymake): Fix use of @itemize and @pxref.  Fix
punctuation and markup.
(Backend exceptions): Use @emph instead of @dfn.  Add more
indexing.
(Customizable variables, Extending Flymake): Improve wording.
(Flymake error types): Fix usage of @itemize.  Improve wording.
(Backend functions): Fix punctuation.  Fix markup.  Add a
cross-reference to ELisp manual.
(Flymake utility functions): Add a cross-reference to ELisp manual.
(An annotated example backend): Fix punctuation and typos.
(Flymake mode, Running the syntax check)
(Navigating to error lines, Backend exceptions)
(Customizable variables, Flymake error types, Backend functions)
(Flymake utility functions, Proc customization variables)
(Locating a master file, Locating the buildfile)
(Starting the syntax check process, Parsing the output)
(Interaction with other modes): Fix indexing.  Add index entries
for functions, variables, and concepts.