chan_pjsip: Fix ability to send UPDATE on COLP

When connected_line_method is "invite", we're supposed to determine
if the client can support UPDATE and if it can, send UPDATE instead
of INVITE to avoid the SDP renegotiation.  Not only was pjproject
not setting the PJSIP_INV_SUPPORT_UPDATE flag, we were testing
that invite_tsx wasn't NULL which isn't always the case.

* Updated chan_pjsip/update_connected_line_information to drop the
  requirement that invite_tsx isn't NULL.
* Submitted patch to pjproject sip_inv.c that sets the
  PJSIP_INV_SUPPORT_UPDATE flag correctly.
* Updated pjsip.conf.sample to clarify what happens when "invite"
  is specified.

ASTERISK-27095

Change-Id: Ic2381b3567b8052c616d96fbe79564c530e81560