bug#33382: 26; doc string of `abbrev-prefix-mark'

[Drew Adams]

(Minor)

The doc string says that the command inserts a temporary hyphen,
temporary until the intended abbrev expansion occurs.

It might be better to say that it inserts a hyphen, and to add that if
the command is later followed by an abbrev expansion then this hyphen is
removed.  That clearly says what _this_ command does, and separates that
from other changes that might come later from other commands.  (If you
don't follow `M-'' with abbrev expansion then the hyphen is not
removed.)

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.16299
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''