Update MH-E's documentation about HTML renderers

Update MH-E's documentation about HTML renderers

* doc/misc/mh-e.texi (HTML): Remove the footnote with the minimum Gnus
version (we are no longer trying to support multiple Emacs
releases). Sort the table of HTML renderers by name (the previous
ordering was based on a 10-year-old survey). Add shr and gnus-w3m to
the table. Remove the entry for w3 (no longer available).  Update
existing entries so that they are more consistent about what features
are discussed, and to reflect recent testing (Debian 8). Small tweaks
to existing text.