holiday-easter-etc incompatibility and docs

[Dave Love]

I forgot to complain about this when I fell over it originally:  the
arg list of `holiday-easter-etc' has changed incompatibly, causing
existing holiday customizations to fail, as noted in NEWS.  If there's
a reason it can't use the old behaviour if called with no args, I
think the new version should be renamed rather than breaking users'
customizations.  (No complaints about having the new functionality
somehow.  It's needed for British bank holidays, at least, and it
might be useful to mention it in (elisp)Holiday Customizing.)

Also the new doc string is unclear.  I think it should say that the
STRING arg is just for display -- it doesn't have to match some
wired-in name, as you might think.  It should probably say that N can
be negative, which is presumably the most common case.

The doc for the holiday functions in general is misleading about what
they return, which is important if you're programming customizations.
When they're for a single date, they actually return a list of a
single pair of the date and a string -- not just the date as the doc
implies.