Doc: Add docs for the FilePathWatcher class to fix a link to it

Change-Id: Ic9f0aea828e423118b74455a40ca7210b73e3284
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/filepath.cpp

@@ -330,6 +330,14 @@ bool FilePath::equalsCaseSensitive(const FilePath &other) const
     return equals(*this, other, Qt::CaseSensitive);
 }
 
+/*!
+    \class Utils::FilePathWatcher
+    \inmodule QtCreator
+
+    \brief The FilePathWatcher class watches a file at a path for editing,
+    renaming, or deletion.
+*/
+
 /*!
     Returns a FilePathWatcher for this path.