* lisp/help-mode.el: Use lexical-binding.
(help-mode-map, help-mode-menu, help-mode-setup)
(help-mode-finish): Make spelling of "Help mode" consistent throughout
the doc strings (also making it consistent with the spelling of "Help
mode" used in the Elisp manual).
(help-do-xref): Re-indent to make the else-branch easier to see.
* test/lisp/help-mode-tests.el: New file with tests for help-mode.el.
