DOCS-2580: update positional projection operator page organization

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-2580: update positional update operator page organization

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-2580: add note about negations and the positional projection operator

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