DOCS-1871 best practices for naming config servers

Signed-off-by: Sam Kleinman <samk@10gen.com>
minor: fixing cross reference
DOCS-1871: edits
DOCS-1632: Clarify to which size metric the limit for sharding an existing collection applies

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1632: edits