Change wording in docstring

Upon reading the docstring for `QuerySet.sort()`, I decided that the
wording was a bit misleading. I feel this wording is a bit clearer.
Typo

The period was missing at the end of the docstring summary line.
Add _sorting attribute to Pipeline

When I was originally adding the attributes to `Pipeline` I seem to have
forgotten to add `_sort`. I'll pretend it was because I wanted to make
sure I implemented it correctly. Really I think I just forgot.

`Pipeline._sort` is handled as `bson.son.SON` because Python's
`OrderedDict` wasn't added to the `collections` module until 2.7. `SON`
is an extension of `dict` adding a few extra features, most noticeably
retaining the insertion order of keys.

References #22