emacs: Support overriding help and describing prefix action

Traditionally, function documentation strings are intended primarily
for programmers, rather than users.  They're written from the
perspective of calling the function, not interactively invoking it.
They're only ever displayed along with the function prototype (and
often refer to argument names).  And built-in help commands like
`describe-bindings' show the name of the command, not its
documentation.

The notmuch help system is like `describe-bindings', but tries to be
more user-friendly by displaying documentation strings, rather than
Elisp command names.  For most commands, this is fine, but for some
the "programmer description" is inappropriate for interactive use.
This is particularly noticeable for commands that take an optional
prefix argument.

This patch adds support for two symbol properties: notmuch-doc and
notmuch-prefix-doc, which let a command override its interactive
documentation and provide separate documentation for its prefixed
invocation.  If notmuch-prefix-doc is present, we add an extra line to
the help giving the prefixed key sequence along with the documentation
for the prefixed command.