bug#37178: 26.2; doc strings of `eshell' and `eshell-buffer-name'

[Drew Adams]

> Maybe it could be made more clear, but I don't see how if we also want
> to keep it sufficiently succinct.

If the behavior, when there is no such existing
buffer, is to just create a buffer with a new
number, maybe just say that, instead of saying
that we "create it", where "it" is a "session
with that number".

IOW, say what it does: you get a new buffer,
but we say nothing about the particular number
it gets.

Another possibility is to change the behavior,
to actually create a session with that number
(why not, if that's what the user really asked
for?): create the buffer with number <42>.

Anyway, I'm OK with what you have now.  Thx.
Mentioning these things mostly for completion.