Clarify 'regexp-opt' match ordering

[Mattias Engdegård]

As mentioned elsewhere, 'regexp-opt' normally returns a regexp that always 
matches the longest string in the given list if more than one could match. This 
is not documented, but it seems very likely that callers often rely on this 
behaviour even if they don't actually give it much thought.

In other words: it's a behaviour that is useful and cannot be changed, so it 
should be documented properly. Proposed patch attached.

0001-More-precise-regexp-opt-documentation.patch
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