PJSIP: Update transport method documentation

This updates the documentation for the 'method' configuration option to
be more verbose about the behaviors of values 'unspecified' and
'default'. They do exactly the same thing which is to select the
default as defined by PJSIP which is currently TLSv1.

Review: https://reviewboard.asterisk.org/r/4264/
........

Merged revisions 430145 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@430146 f38db490-d61c-443f-a65b-d21fe96a405b