Fix documentation for Revision::getComment and WikiPage::getComment

Fix documentation for Revision::getComment and WikiPage::getComment

This mismatch between the documentation and how the code behaves was
introduced via Ia4c20a9 in December 2017. The getComment() methods have
never been documented to return null, but now do.

I tried to fix this both ways (see I32202df for the other approach), but
decided it's easier and cleaner to keep the null. Reasoning:

* The code behaves like this for more than a year already. Removing the
  null would possibly be as desruptive as when it was introduced.

* Removing the null breaks Wikibase.

* I think returning null is the right thing to do.

I did a brief check of all callers of these two methods. I found a few
that might misbehave a little bit when they are confronted with null, e.g.
in MobileFrontend or Echo. In the vast majority of cases the null will be
silently casted to an empty string and be fine. In a few cases a "the
comment is empty" message might disappear, and the null be shown in the UI
instead, possibly resulting in an empty, confusing HTML element. I would
argue this is an issue with the frontend code then. It should not expect
to have access to all comments. Some might be supressed.

Bug: T174025
Change-Id: I0e1ff9686d1d875812460631c29330c398e74bcf