API: update documentation and suppress warning when parsing summary

The documentation states that if contentmodel and title are not given,
but text exists, "wikitext" will be the default model. However, the
actual code will show a warning if do so. This patch fixes the inconsistence
by removing some parts of the documentation which encourages giving
only text as the parameter and corrects the example. The patch
also suppresses the warning that contentmodel must be given when
summary parameter is given.

bug: 60192
Change-Id: I0f41c83763fbb5551aa3ceaff59c1f8fe38310be