Snapshot FAQ update

[nstewart]

Add answer to common questions about snapshotting

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[nstewart]

@kay-kim @sjwiesman our docs here say to scale up via an alter. But I know we are doing a bunch of scaling up via adding a replica. What do we actually want to say to do for speeding up a snapshot?

[kay-kim]

@nstewart -- so, I was under the impression (from the docs)

Increasing the replication factor does not increase the cluster’s work capacity.

[kay-kim]

Could you clarify about

But I know we are doing a bunch of scaling up via adding a replica

[nstewart]

My understanding is that you can add a replica with a different size, which does make a difference cc @sjwiesman