Update $sum operator reference documentation

Remove a confusing part of the sentence referencing a non-existent example

Signed-off-by: kay <kay.kim@10gen.com>
trivial remove references to non-existent use cases
DOCS-2904 pluralize document to documents
fix copy + paste error from removing an include file
Fix rs.stepDown() method syntax

Signed-off-by: kay <kay.kim@10gen.com>
DOCS-2913 make the drop method consistent with drop command
DOCS-2878 filterSet field in explain output
DOCS-2895 add label for dochub link
DOCS-2910 ensureIndex with name but diff key spec
DOCs-2919: fixing maxDistance
DOCS-2919 maxDistance should be in near doc not geometry doc
DOCS-2881: correct casing of ObjectId
DOCS-2856: moving saslServiceName documentation to parameters rather than runtime options
DOCS-2919 maxDistance should be in near doc not geometry doc
minor: remove cruft from mongos page