Reword descriptions of HTML visual editor toolbar

http://edx.readthedocs.org/projects/edx-partner-course-staff/en/latest/creating_content/create_html_component.html#the-visual-editor

this came up in training: for HTML component editing toolbar, what is “code block”?
One of the icons has a mouseover tip of "code block" although what it does is reformat the text in a monospace font. We don’t actually define this in the doc. We provide an image with callouts for the icons on the toolbar, but “Format selected text as code.” is the description. Need to specifically linking the “code block” mouseover tip to this description, and identify what it is that causes the text to be "formatted" as code.

Do this consistently for all of the callout descriptions.