bug#34303: 26.1; Doc of `date-to-time'

[Drew Adams]

The doc could be a bit clearer.  The doc string says that the arg is
DATE, a string representing a date-time.  The Elisp manual calls the arg
STRING and says it is a time-string.  Both should say clearly just what
string formats they expect as arg.

In GNU Emacs 26.1 (build 1, x86_64-w64-mingw32)
 of 2018-05-30
Repository revision: 07f8f9bc5a51f5aa94eb099f3e15fbe0c20ea1ea
Windowing system distributor `Microsoft Corp.', version 10.0.17134
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''