Code tidy and fix broken documentation links.

doc/docslib.php

doc/en/Authoring/Answer_tests.md

doc/en/Authoring/CASText.md

doc/en/Developer/Documentation.md

doc/en/Installation/index.md

doc/maintenance.php

edit_stack_form.php

locallib.php

questiontestform.php

stack/input/textarea/textarea.class.php

stack/questiontestresult.php