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.
