Override overflow: hidden; for <h3>s in the command help dialog

When this dialog is not isolated, MediaWiki's overflow: hidden;
rules for headings cut off part of the text in the <h3>s.

Compensates for Id425c56d9d24 in VE core.

Change-Id: Id9f6bd2bdbed0c6fe57e897c2415c3eceff900a6
Merge "Override overflow: hidden; for <h3>s in the command help dialog"