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