Fixes T10381. When we converted to PHUIRemarkupView, some instructional text got linebreaks added when it shouldn't have them (the source is written in PHP and wrapped at 80 characters, but the output should flow naturally).
Fix this so we don't preserve linebreaks.
This also makes PHUIRemarkupView a little more powerful and inches us toward fixing Phame/CORGI remarkup issues, getting rid of PhabricatorMarkupInterface / PhabricatorMarkupOneOff, and dropping all the application hard-coding in PhabricatorMarkupEngine.