Updates to bring the scaling documentation (Passenger and multi-master) up to date with current Puppet versions
Style/usage: lowercase puppet when referring to puppet master servers
Modify passenger docs for structure/flow/style

- Use $ sudo <command> style throughout, to match several similar docs.
- Rearrange headers for a more legible hierarchy.
- Explain one or two things more thoroughly.
Multi-master: De-emphasize SRV records, and flag them as an experimental feature

This commit re-orders the options for distributing agent load and CA
traffic to put SRV records at the bottom. They're cool and they work,
but they have some pitfalls the user needs to know about, and the
feature could use some love before we can call it the preferred method.
Multi-master: Clarify a few things

I made a mistake in describing cert traffic as beginning with
/certificate -- thinking like auth.conf instead of thinking like Apache.
This commit clarifies that, as well as tells you basically all you need
to know for expanding the inventory service w/ PuppetDB.
Merge branch 'shanemadden_scaling'