Change some headings in the docs

There are some headings on C++ and QML reference pages that concern both signals and signal handlers. Currently, the headings include the phrase "Signal Handlers," although many times what is documented there are signals, not signal handlers. Sometimes, however, it is signal handlers that get documented. The problem is we don't all the documenter to specify which one he is documenting. That's the real problem, and it requires adding a \qmlsignalhandler command to qdoc, or something like that. But for now, this task just changes the headings in the documentation back to "Signals" because that is a bit less confusing.