bug#39376: [PATCH] Throw error when calling reverse-region on a single line

[Eli Zaretskii]

> From: Philip K <philip@warpmail.net>
> Date: Fri, 31 Jan 2020 22:26:24 +0100
> 
> The current implementation of reverse region will delete everything
> following the current line, if the region marks only a part of a
> line. This is because the algorithm implemented below accidentally
> deletes all following lines, and fails before being able to reinsert
> them.
> 
> This patch sends an error (could be changed to an user-error) in that
> case, as it makes no sense for a single line to be reversed.

Thanks.  I installed a slightly different change, because the command
would still delete the following lines if only BEG or END (bit not
both) was not at a line beginning/end.

I also fixed the doc string and the comments there.