> Or a more prompting style
>
> "If INCLUDE-DIRECTORIES is non-nil, also include directories whose names
> matches REGEXP in the returned list."

I did consider this style. But, it is slightly harder to parse. For
example, it is easy to misparse the sentence as "also include
directories (whose names match REGEXP in the returned list)" when the
correct parsing should be "also include (directories whose names match
REGEXP) in the returned list".