DOCS-2000 update restrictions on database names
DOCS-1997: clarifying gle metrics
DOCS-1547: Change details around Tag Aware sharding and Chunks that span tags

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-442 add example of sharded cluster backup and restore

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-442: edits