Consistent usage of codename/passphrase in Source Guide

[ptrubatch]

In the reading through the Source Guide as someone who is new to SecureDrop, I I found the term "passphrase" to be a bit confusing because "codename" is otherwise used consistently in this section.

Issue #2176 addresses these types of inconsistencies elsewhere, but I think it'd be helpful to make this codename/passphrase terminology a little clearer in the Source Guide. Even adding a parenthetical, e.g. (also known as a passphrase), after the first mention of codename would help.