* Escape some text that shouldn't be linked, e.g. IMAP.
* Link some text that should be linked:
  * removing "+" to let classes link
* Remove `Net::IMAP::` prefix from response data class names.  The
  redundant prefix breaks up the reading flow of the text.  Examples
  will still use fully-qualified names, and readers should generally be
  able to see that the classes/modules are in Net::IMAP, from context.
* Add headings and convert text to headings.
* Reduce headings inside method docs to h5 (call-seq renders like h4).
* Add <tt> or "+" to code or verbatim text.
* To match the tweaks to rdoc's darkfish template, convert definition
  lists to "note" vs "label" style lists for table vs lists.
* Add ">>>" with <em> to highlight important "aside" notices.
* Add :nodoc: to ResponseErrors.

* Document capabilities at the class-level, linking to #capability for
  details.
* Add "Basic IMAP4rev1 capabilities" and "Using IMAP4rev1 extensions"
  sections to the #capability rdoc.
* Add relevant capabilities to the rdoc for every command extension.
* Update existing capability docs to the same consistent format.
* Add TODO comment for unsupported LIST-EXTENDED

Every IMAP command method now links to the RFC section that defines it.

There are a lot of IMAP commands!  This makes it easier to navigate
between them.  Hopefully this makes it easier to learn the IMAP4rev1
protocol and its extensions, as well.

* Link to UIDPlusData for commands that can return it:
  `#append`, `#copy`, and `#move`

* Update `#uid_expunge` to avoid "UID set", which has slightly different
  semantics from "sequence set of UIDs".

* Add paragraph to class-level docs, about expunge messages.
* Add explanation to #noop docs, explaining EXPUNGE responses
* Update #uid_expunge docs with method link instead of command name.

The documentation on these methods is meant to complement a new
SASL::Authenticator base class and new documentation on each of the
individual authenticator classes.  See #78 and #82.

That base class is added in another PR (#78), but the documentation for
these methods can be updated without it.

Also, somehow I misremembered `LOGINDISABLED`: it only applies to
`LOGIN`, not `AUTHENTICATE`!  This fixes that error.

Add a note about the implicit UID message data item.

Makes it just a little bit simpler.

This "What's here?" section is based on the new documentation style for
many core ruby classes.  It isn't purely a list of methods; paragraphs
of explanatory text are interspersed.

*Many* "TODO" comments are added too, hidden from rdoc by `--` and `++`.

Without resetting to the default section, code that's loaded later may
be documented in the wrong place