Doc: Fix template information for QObject::findChildren

When QDoc parses a project, it parses the source code to extract the
user-provided documentation and perform sanity checkings based on the
code itself on it.

When QDoc parses an "\fn" command as part of this process, it tries to
understand, based on its intermediate representation built on the
information extracted from the code-base, which "documentable element"
the "\fn" refers to.

When QDoc performs this "matching" process, it takes into consideration
only a certain amount of information.
For example, no checking is performed over the template declaration of a
callable.

Due to some upcoming documentation, where two callables are
indistinguishable to the current process, as they differ only in their
template declaration, QDoc will start to take into consideration the
template declaration of a callable when matching.

This implies that an "\fn" command should now provide information
parity, with regards to template declaration for callables, with the
code-base so that QDoc can perform the match correctly.

The documentation for `QObject::findChildren` is
not in sync with the intended target template declaration.

Hence, add the missing information to the relevant "\fn" commands.

Task-number: QTBUG-118080
Change-Id: I325008c22e82c002afe8cbeaa847af7a28277a0b
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

src/corelib/kernel/qobject.cpp

@@ -2059,7 +2059,7 @@ void QObject::killTimer(int id)
 */
 
 /*!
-    \fn QList<T> QObject::findChildren(const QRegularExpression &re, Qt::FindChildOptions options) const
+    \fn template<typename T> QList<T> QObject::findChildren(const QRegularExpression &re, Qt::FindChildOptions options) const
     \overload findChildren()
 
     \since 5.0