Update BView and BStringView docs

... based on Adrien's suggestions.

* Add \warning back to BView::ResizeToPreferred(), add \remark to highlight
  methods only appropriate to use as part of a BLayout.
* Clarify BView::SetFont() method and refer to BFont docs.
* Clarify BStringView::SetText(), Text(), and Draw().
* add field descriptions for BStringView::Instantiate().