lib: reword comment about XFOLDER: prefix

I believe the current one is misleading, because in my experiments
Xapian did not add : when prefix and term were both upper case. Indeed,
it's hard to see how it could, because prefixes are added at a layer
above Xapian in our code. See _notmuch_message_add_term for an example.

Also try to explain why this is a good idea.  As far as I can ascertain,
this is more of an issue for a system trying to work with an unknown set
of prefixes. Since notmuch has a fixed set of prefixes, and we can
hopefully be trusted not to add XGOLD and XGOLDEN as prefixes, it is
harder for problems to arise.

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