Document that auth-user-pass may be inlined

Document that auth-user-pass may be inlined

Commits 7d48d31b, 39619b7f added support for inlining username
and, optionally, password.
Add a description of its usage in the man page.

Github: resolves OpenVPN/openvpn#370

Change-Id: I7a1765661f7676eeba8016024080fd1026220ced
Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20240220175215.2731491-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28284.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>