Warn that the "tags" whitelist is order sensitive

[dantman]

> langtag.lookup(['en', 'en-CA'], ['en-CA'])
'en'
> langtag.lookup(['en-CA', 'en'], ['en-CA'])
'en-CA'

If a root locale like en comes before subtags like en-CA in the tags whitelist langtags will never reach the more specific locale, even if the order of the range user locale list indicates that the more specific locale is preferred.

Documentation should probably warn that you need to put any shorter locales after the more specific locales in the tags list.

This is important because some locale libraries (cldr, Globalize, ...) omit an en-US locale and instead make en equivalent to en-US. So a lookup for ['en-US', 'en-CA'] would actually be ['en', 'en-CA'] and if not re-ordered would result in US English always being used even if other non-US English locales are also supported.