Document repo update behavior

Created by: beyang

We have had inquiries from customers and members of our own team about how frequently Sourcegraph updates repositories and if there is a way to control this behavior. Examples:

From $CUSTOMER:

Need a quick reposes on this if you have solution.

We are using “Other Git repository hosts” to let SG clone the repos and then it check for updates.

How frequent it tries to pull the repo updates ?

Can we reduce the frequency to reduce load on repo host ? And also if we need to temporarily shut-off the index update ?

From @ryan-blunden:

What is the expected delay from when Sourcegraph syncs a new commit, to that change being indexed and findable? (Thread)

We have documentation for specific fields like repoListUpdateInterval and gitMaxConcurrentClones, but AFAICT no high-level documentation.

Proposal: Add to our docs (1) FAQs about repository update behavior and (2) a description of the mental model users should have when reasoning about repository update behavior.