Make SSPI documentation clearer

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/sspi-auth.html
Description:

The [current SSPI
documentation](https://www.postgresql.org/docs/current/sspi-auth.html)
reads:

"SSPI authentication only works when both server and client are running
Windows, or, on non-Windows platforms, when GSSAPI is available."

I interpret that phrase like this:

* there's a case where both server and client are running Windows
* there's a case where both are running non-Windows

What about mixed cases? When the client is non-Windows, then can it use
SSPI? No, AFAIK not. So I'd suggest to make that phrase above clearer and
completely explicit:

"SSPI authentication works when both server and client are running
Windows.

When the server is on a non-Windows platform then the server must use GSSAPI
if it wants to authenticate the client either via Kerberos or via Active
Directory. A client on a Windows platform that connects to a non-Windows
Postgresql server can either use SSPI (strongly encouraged) or GSS (much
more difficult to set up) if it wants to authenticate via Kerberos or Active
Directory. A client from a non-Windows platform must use GSS if it wants to
authenticate via Kerberos or Active Directory."

Thanks a lot for everything Postgres to you who is reading a considering
this change!