doc: align doc/api/tls.markdown with style guide

Brings tls.markdown into alignment with the node.js
styleguide, specifically regarding the use of
personal pronouns. Also, fixes various typos,
punctuation errors, missing definite/indefinite
articles and other minor grammatical issues.

PR-URL: nodejs#5706
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>