Mark customizable variable as changed

`flycheck-highlighting-mode` has a new default value in the upcoming
release.
Add Info and Website links to customization groups
Add additional tests for error regions
Document `flycheck-error-sexp-region` in manual
Add a convenience constructor to Flycheck error struct
Remove superfluous `flycheck-error-region`
Try to reasonable handle error lines beyond EOF

Make `flycheck-error-column-region` return the very last column in the
file if the error line is beyond EOF.
Add test for `flycheck-error-pos`