Add documentation to checkers

Add documentation for syntax checkers.

Remove documentation in `README.md`, and attach documentation to syntax checkers.  Add `flycheck-describe-checker` to show the documentation of a checker.

As part of this all checkers have been "de-qualified".  Since the checkers are not used as variables or functions by Flycheck, but only as place for (qualified) properties, we can safely remove them from the `flycheck-checkers` namespace which is more convenient in dealing with checkers.