Provide more details for new committers on Bugzilla account merging

Update Bugzilla section of new committer's guide with more detailed
description of Bugzilla account merging and add reference to Kerberos
password generation/change. Based on emails bugmeister@ team receives these
items are the most common sources of confusion for new FeeBSD committers
when it comes to bugtracker.

Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D12828