Clarify replication configuration documentation

https://forums.foundationdb.org/t/locking-coordination-state-after-process-removal/1512/5 nicely explains how the phrasing in https://apple.github.io/foundationdb/configuration.html#single-datacenter-modes could be interpreted in unintended ways. We should find some less misinterpretable text to use, most of which has already been suggested in the forum post.

Read the single-datacenter modes section of configuration.html and the linked forum discussion, especially the suggested replacement wording. Revise the replication configuration text so its meaning cannot be interpreted in the unintended way described there, then verify the rendered documentation communicates the intended behavior.

Written by the indexing model from the issue text.