Add clarity to "The filename syntax must be only /vsistdin/"

[jidanni]

Feature description

In https://gdal.org/user/virtual_file_systems.html

we see

The filename syntax must be only /vsistdin/.

It would be great if it instead said

The filename syntax must be only /vsistdin/, not e.g.,
/vsistdin/path/to/f.csv .

But maybe that's not what it is trying to say. Hard to tell! Thus I
have no PR to submit.

And, a little further down it goes on to say,

The "/vsistdin?buffer_limit=value" syntax can also be used.

which rather undermines what it first said.

Later in the page it would also be nice if

The filename syntax must be only /vsistdout/.

said

The filename syntax must be only /vsistdout/. Not e.g., .....

[rouault]

The filename syntax must be only /vsistdin/, not e.g.,
/vsistdin/path/to/f.csv .

yes, that's what it is meant. But that should be kind of obvious for anyone a bit familiar with the concept of standard input.

And, a little further down it goes on to say, The "/vsistdin?buffer_limit=value" syntax can also be used.

That's a later addition to the original "/vsistdin/" only syntax. This is a rather advanced mode, that said

Feel free to submit an enhancement reflecting that if you wish.