Document that we do not balance replicas among multiple stores per node

[rmloveland]

For details, see cockroachdb/cockroach#6782

tl;dr: we do not rebalance replicas among multiple stores on the same node.

Estimated scope of work:

• Update cockroach start docs where it talks about multiple stores per node
• Seek out and update other mentions of this feature
• Update them all with a note (perhaps an include)

Related issues:

• Add more info around running a node across multiple stores #3197
• Start a Node Doc Update -- Handle Multiple Data Stores #4684

[ericharmeling]

This appears to have been resolved. Please re-open if I'm mistaken.

[rmloveland]

Probably (?) but I need to look more closely at this to be sure - reopening so it doesn't fall off my radar.

[ericharmeling]

Going through the known limitations issues as I'm documenting 21.1 known limitations, and this appear to have been resolved by cockroachdb/cockroach@ed34965. I'm going to close for now, if that's okay?