Update sharded-cluster-internals.txt

User had confusion because balance threshold resets when any chunk migration fails.

Signed-off-by: Sam Kleinman <samk@10gen.com>
change image width size in metadata and fix rs.add

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1525 fix alignment for proper rendering

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