Add docs and instructive comments to sparql-mode.el

[candera]

Background

People generally expect a major mode file to have comments at the top of the file explaining how to use it, and to have reasonable documentation strings. sparql-mode currently lacks both of these things.

Acceptance Criteria

• The documentation strings for all functions is updated in conformance with emacs conventions.
• The documentation string should contain {KEYMAP}
• The top-of-file comment is updated in conformance with emacs conventions.