org-agenda-filter-by-tag doco

[Phil Hudson <phil.hudson@iname.com>]

I'd like to expand the docstring of command `org-agenda-filter-by-tag`
to document the Elisp equivalent of interactive prefix arguments. By
trial and error and guesswork I determined that I could emulate `C-u
C-u` by passing `'(16)` as the second argument
(`strip-or-accumulate`). I'd like to add text explaining this,
generalized to all prefix combinations.

I just thought I'd mention it on the list first, to get any thoughts
and advice that might be useful. For instance, currently there is a
paragraph for each prefix combination (`C-u`, `C-u C-u` and `C-u C-u
C-u`). Should I add Elisp calling doco to each, or should I expand the
subsequent paragraph that deals specifically with Elisp calling? I
tend to favor the latter as least confusing, but that might be a
matter of convention or opinion that others are better qualified to
comment on.

A further point: I already knew from prolonged usage that `C-u`
combinations evaluate numerically as powers of 4, but a newbie might
not. Should we explain this? I don't think it would be appropriate,
but again I'm open to advice. If not, should we at least link to the
explanation in the Elisp manual? I know we don't usually bother, but
since outcomes differ substantively in this case, maybe we should
here.