Today the docs say that the low watermark has no effect on any shards that have
never been allocated, but this is confusing. Here "shard" means "replication
group" not "shard copy" but this conflicts with the "never been allocated"
qualifier since one allocates shard copies and not replication groups.
This commit removes the misleading words. A newly-created replication group
remains newly-created until one of its copies is assigned, which might be quite
some time later, but it seems better to leave this implicit.
f501a4b2b5