Where to get sip.conf manual?

Where one can read about precise meaning of all parameters of sip.conf file?

For example, I am interested to know, why sections are parametrized with context parameter? I think that context parameter is sensible for incoming calls while these sections are for outcoming. What if several sections describing one provider will have different contexts? How Asterisk will determine which one to use?

The source code is the only defnitive reference.

One cannot tell from looking at sip.conf whether or not incoming calls will come in on a particular device.

The first matching entry will be used. This can cause real problems if the same IP address is used for all your accounts. You may need the pending change in issues.asterisk.org/view.php?id=16585 in difficult cases.