The Halibut manual contained at least one instance of two index

The Halibut manual contained at least one instance of two index
terms (intentionally) differing only in case, which were being
silently folded into one by the case-insensitive index tag
comparison. Halibut now warns in this situation (but then folds them
anyway, which I think is better than silently generating an index
containing many case-distinct forms of the same word - I imagine
it's very easy to do that by mistake). The manual has been fixed to
explicitly define distinct keywords (in the case I spotted and in
five other cases picked up by the new warning!), and also documents
this issue and how to work with it.

git-svn-id: svn://svn.tartarus.org/sgt/halibut@4279 cda61777-01e9-0310-a592-d414129be87e