Restore ability to disable cross references for inline `tt`

[nevans]

The problem

Prior to #1240, tt text (created by +word+, <tt>some text</tt>, or <code>some text</code> would not automatically convert cross-references into links. Now it does.

This is a huge improvement! However, sometimes we don't want a monospace text to automatically cross-reference.

Outside of tt text, we can disable automatic linking by prefixing \. Unfortunately, \ seems to disable all inline attributes?

My proposal

Leave +word+ and <tt>some text</tt> the exact same as they are today. This is a big improvement. \ should still disable all inline attributes for +\WORD+, as it + only works with words (and \ isn't a word char) and it also allows the +s to be rendered.

But make one or more of these changes, in my personal order of preference:

1. <code>some text</code> should go back to the way it used to behave, with no cross-linking. The semantics or tt vs code should make it easy to remember.
2. Inside <tt> (and <code>) tags, \ should be able to disable linking without disabling any of other inline styles.
3. Alternatively: maybe add support for <pre>...</pre>?

Two examples

Here's one example, which may not be the best example, but it's the most recent one I've run up against:

module Net
  # Implements a \IMAP protocol client.
  #              ^^^^^
  #                 \
  #                 This refers to the protocol, not the class,
  #                 so we've escaped it.
  class IMAP
    # A message flag that is set on deleted messages, before they are expunged.
    DELETED = Flag[:Deleted]

    # Sends a +STATUS+ command for a +mailbox+ and returns a hash of status
    # attributes and their values.
    # 
    # +attrs+ lists the requested status attributes, as an array of strings.
    # 
    # Supported attributes:
    # +MESSAGES+::
    #   number of messages in the mailbox
    # +DELETED+::
    #   Links to Net::IMAP::DELETED, which is a slightly different concept.
    #   That's a message flag, this is a status attribute name.
    # <code>DELETED</code>::
    #   Also links to Net::IMAP::DELETED.
    # +\DELETED+::
    #   Not a link, not monospace font, and renders both of the +s too.
    # <code>\DELETED</code>::
    #   Not linked, not monospace, but the code tags aren't rendered
    #
    # <tt"DELETED"</tt>::
    #   The arguments are provided as strings, so _technically_ this is a
    #   potential solution for _this_ example.
    #
    #   But the status attribute names are referenced in other ways elsewhere,
    #   and they refer back to this rdoc.  And, because the \IMAP protocol
    #   distinguishes between <tt>"quoted text"</tt> and +ATOM+ values, the
    #   documentation avoids using quotation marks for protocol atoms, even
    #   when they are provided as string arguments.
    #
    def status(mailbox, attrs)
      mailbox = String(mailbox.to_str)
      command "STATUS", mailbox, attrs, handle_untagged: ->req {
        req in name: "STATUS", data: StatusData[mailbox: ^mailbox]
      }
    end
  end
end

I've come across other examples in Net::IMAP, which might be better than this one.

But none of them are as good as the example I found in this repository when I went looking to see if there's a solution. Nearly every example in the "Links" section of markup_reference/rdoc.rdoc is broken now (the current rendered version). For example:

=== Class Links

- On-page: <tt>RDoc::Example::ExampleClass</tt> links to RDoc::Example::ExampleClass.
- Off-page: <tt>RDoc::Alias</tt> links to RDoc::Alias.

This is currently rendered as:

Which is not at all helpful! And it's the same for many many other examples on that page.