bug#21467: 25.0.50; doc of `set-char-table-extra-slot'

[Drew Adams]

Please consider mentioning that N is zero-based, not one-based.

(elisp) Char Tables does say that the value of property
`char-table-extra-slots' is 0 to 10, so a reader can figure it out in
the manual.  But it would still help to state it explicitly for N in the
description of functions such as `char-table-extra-slot' and
`set-char-table-extra-slot'.  And the doc string says nothing about the
starting value for N.

In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2015-08-16 on LEG570
Bzr revision: f7ee23e587b01f179284b5554c67d579a2def676
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --host=i686-pc-mingw32 --enable-checking=yes,glyphs'