Clarify the distinction of unset scalar fields vs unset message fields.

[smolkaj]

There is a subtle distinction between the treatment of default-valued message fields vs default-valued scalar fields. This has bitten me and others at Google one to many times, so this PR clarifies the distinction.

This PR is not intended to introduce any semantic changes.

[antoninbas]

let's hold off on merging this, I think @jafingerhut wanted to take a look over the weekend

[smolkaj]

Sounds great, no rush. Thanks everyone for the reviews!

[jafingerhut]

As of commit 6 on this PR, I see all "master" occurrences replaced with "primary", so looks ready to go to me.

[smolkaj]

Thanks!