[Merge to XFA] Minor documentation updates for fpdfview.h.

This does some minor copy editing and formatting of the documentation in the
fpdfview.h header.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1411863004 .

(cherry picked from commit 19169dae5bf92813b7419793f61dfca8b4fb9124)

Review URL: https://codereview.chromium.org/1420593002 .
1 file changed