Minor changes to MDCAppBar documentation.
Minor changes to MDCAppBar documentation.

Reviewers: #mdc_ios_owners, ajsecord

Reviewed By: #mdc_ios_owners, ajsecord

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D281

Eccelor/material-components-ios,samnm/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,sigito/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,willlarche/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,chriscox/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,material-components/material-components-ios,samnm/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios