PJSIP: PJSIPNotify - Strip content-length headers and add documentation

Documentation for how to add custom headers/content to notifies created
with the PJSIPNotify manager action was a little sparse and it also
wasn't vetting application of Content-length headers like its chan_sip
equivalent was (so two Content-length headers could be applied... and
PJSIP determines the content length anyway, so it just opens people up
for error). This patch also flips the variable order so that the
variables are interpreted in the same order as they are put in the AMI
action.

Review: https://reviewboard.asterisk.org/r/3587/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@415658 f38db490-d61c-443f-a65b-d21fe96a405b