bug#20419: 25.0.50; doc string of `recenter-last-op`

[Drew Adams]

The doc string says that these are the possible values:

"`top', `middle', `bottom', integer or float numbers."

It should also mention `nil', say what `nil' means, and perhaps even say
that `nil' is the default value.

A user reading things like the binding of this variable to nil in
`perform-replace' is not helped by consulting the variable's doc.



In GNU Emacs 25.0.50.1 (i686-pc-mingw32)
 of 2014-10-20 on LEG570
Bzr revision: 118168 rgm@gnu.org-20141020195941-icp42t8ttcnud09g
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --enable-checking=yes,glyphs CPPFLAGS=-DGLYPH_DEBUG=1'