Update exists.txt
DOCS-1886: fixing syntax error
edits to $exists
DOCS-1182 shard key restrictions

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1182: shard key restriction edits
The $mul example corrected

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

Signed-off-by: Sam Kleinman <samk@10gen.com>
additional GLE return values

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