configs: Improve documentation for bandwidth in iax.conf.

configs: Improve documentation for bandwidth in iax.conf.

This improves the documentation for the bandwidth setting
in iax.conf by making it clearer what the ramifications
of this setting are. It also changes the sample default
from low to high, since only high is compatible with good
codecs that people will want to use in the vast majority
of cases, and this is a common gotcha that trips up new users.

Resolves: #425