Copy edits for typos

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update write-operations.txt

Fix a slightly jarring grammatical error (criteria is plural).

Signed-off-by: Sam Kleinman <samk@10gen.com>
Fix mismatched field name in documentation

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-2265 new built-in roles