res_pjsip: Update authentication realm documentation.

Using the same auth section for inbound and outbound authentication is not
recommended.  There is a difference in meaning for an empty realm setting
between inbound and outbound authentication uses.

An empty inbound auth realm represents the global section's default_realm
value when the authentication object is used to challenge an incoming
request.  An empty outgoing auth realm is treated as a don't care wildcard
when the authentication object is used to respond to an incoming
authentication challenge.

ASTERISK-26799

Change-Id: Id3952f7cfa1b6683b9954f2c5d2352d2f11059ce