Doc: edit QWebEngineUrlSchemeHandler docs

- Add missing docs
- Edit for grammar

Change-Id: I407818d66f17a354cf3f5051bed5a648bc807424
Reviewed-by: Florian Bruhin <qt-project.org@the-compiler.org>
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>

src/core/api/qwebengineurlschemehandler.cpp

@@ -43,16 +43,22 @@ QT_BEGIN_NAMESPACE
 
 /*!
     \class QWebEngineUrlSchemeHandler
-    \brief The QWebEngineUrlSchemeHandler base class for handling custom URL schemes.
+    \brief The QWebEngineUrlSchemeHandler is a base class for handling custom URL schemes.
     \since 5.6
 
-    To implement a custom URL scheme for QtWebEngine you must write a class derived from this class,
+    To implement a custom URL scheme for QtWebEngine, you must write a class derived from this class,
     and reimplement requestStarted().
 
     \inmodule QtWebEngineCore
 
 */
 
+/*!
+    \fn QWebEngineUrlSchemeHandler::destroyed(QWebEngineUrlSchemeHandler*)
+
+    This signal is emitted when a custom URL scheme handler is deleted.
+*/
+
 QWebEngineUrlSchemeHandlerPrivate::QWebEngineUrlSchemeHandlerPrivate(const QByteArray &scheme)
     : m_scheme(scheme)
 {
@@ -70,6 +76,9 @@ QWebEngineUrlSchemeHandler::QWebEngineUrlSchemeHandler(const QByteArray &scheme,
 {
 }
 
+/*!
+    Deletes a custom URL scheme handler.
+*/
 QWebEngineUrlSchemeHandler::~QWebEngineUrlSchemeHandler()
 {
     Q_EMIT destroyed(this);