FindQt4: Use standard section header underline convention

The FindQt4 module is a CMake Domain object of type "module".
Our convention for domain object document titles is "------"
and domain object document sections is "^^^^^^".  Switch to
these within the FindQt4 documentation.
Merge topic 'FindQt4-doc-section-underlines'

06ab7db3 FindQt4: Use standard section header underline convention