bug#22107: 25.1; Wrong docstring for this-single-command-keys

[Stefan Monnier]

> So how about saying a few words about the meaning of the return value
> of internal-echo-keystrokes-prefix?  I think it's important enough to
> have that at least documented in comments.

It's used as a proxy for "we're in the middle of a command-with-prefix,
which may be using the `prefix-arg' variable or any other variable".


        Stefan