Some doco tweaks for I18N::LangTags::Detect

I had to use I18N::LangTags::Detect last night and when I saw in the
doco that it looks for "several environment variables, including [list
of variables]" I thought "OK, so what other variables are involved that
I'll need to control from my tests?" and had to read the source. One of
the patches attached to this message updates the doco to make it clear
exactly which variables it looks at.

While reading the source I also found an undocumented environment
variable, so there's a little patch for that too.

-- 
David Cantrell

• Some doco tweaks for I18N::LangTags::Detect by David Cantrell