Tune ‘format’ after recent fix

Tune ‘format’ after recent fix

* doc/lispref/strings.texi (Formatting Strings):
* src/editfns.c (Fformat): Format field numbers no longer need
to be unique, reverting the previous doc change since that has
now been fixed.  Also, document that %% should not have modifiers.
* src/editfns.c (styled_format): Improve performance.  Remove
the need for the new prepass over the format string, by using
a typically-more-generous bound for the info array size.
Initialize the info array lazily.  Move string inspection to
the same area to help caching.  Avoid the need for a
converted_to_string bitfield by using EQ.  Cache arg in a
local and avoid some potential aliasing issues to help the
compiler.  Info array is now 0-origin, not 1-origin.