bug#23867: 25.1.50; Doc string containing `\N'

[Drew Adams]

> > Before that "\N" was the same as "N", so there is no point in writing
> > that.
>
> Good point.  Thank you.

It's a valid point, but it is also wrong.

There might be "no point in writing" \N, but there can be a use
for having \N act as N - as it always has.  Doc strings can be
produced by _code_, and that code can escape ordinary characters,
for which the escaping should be a no-op.

There have been very few exceptions to this in the past - very
few characters for which \-escaping is not a no-op.  What's the
rationale for breaking this for N?