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-1365 shardConnPoolStats command