mediawiki.messagePoster: Minor code and docs clean up

mediawiki.messagePoster: Minor code and docs clean up

* Improve and simpify various descriptions.

* Consistently use the imperative mood for the first line of
  method descriptions.

* create(): Refactor to be more stable and resilient.
  - Move variable declarations to inside the Deferred handler to avoid
    fragile code where data is transferred between different promise callbacks.
  - Remove check for 'result.query.pageids'. If it doesn't exist then neither
    does 'result.query' and it would throw either way.
  - Simplify check by asserting [0] directly instead of computing the length.
    Matches the actual code below.
  - Rename local variable 'errorCode' to match documentation of second param
    being 'error'. 'errorCode' is the first parameter passed to reject().
  - Outdent body and change to pattern of error-first returning.
  - Rename unexposed classname to be plain without MW prefix.

Change-Id: If642e94942abcbf7e5aa373fbd83a30d9202f24a