emacs: document/defcustom notmuch-multipart/alternative-discouraged

This variable is important for people who want to change the default
behaviour when displaying multipart/alternative messages.  Previously
it was undocumented.  Add a defcustom to help users and copy some
documentation from the wiki. The usual machinery of re-using
docstrings is a bit tricky to use here because it mangles the example
lisp code, and the link to the info node should not be in e.g. the
html page.

Add a simple test to make sure the switch from defvar to defcustom did
not break something obvious.

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