bug#55984: 26.3; Doc string of `chmod' / `set-file-modes'

[Drew Adams]

Please consider mentioning in the doc string that the command prompts
for the file name and (then for) the modes.  The doc currently says how
the modes are read, but it doesn't say that the command reads the file
name.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19044
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''