This adds MITM and SNI as acronyms, as the documentation already had
them marked up with <acronym>.

While on it, make sure to spell man-in-the-middle with dashes
consistently, and add acronyms for those new terms where appropriate.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CE12DD5C-4BB3-4166-BC9A-39779568734C@yesql.se