fix more dox warnings

doc/main.txt

@@ -55,6 +55,6 @@
  * ns3 namespace.
  */
 /**
- * \defgroup constants
+ * \defgroup constants Constants
  * \brief Constants you can change
  */

src/core/assert.h

@@ -22,7 +22,7 @@
 #define ASSERT_H
 
 /**
- * \defgroup assert
+ * \defgroup assert Assert
  * \brief assert functions and macros
  *
  * The assert macros are used to verify

src/core/callback.h

@@ -458,6 +458,7 @@ Callback<R> MakeNullCallback (void) {
 }
 /**
  * \ingroup MakeCallback
+ * \overload Callback<R> MakeNullCallback (void)
  * \return a wrapper Callback
  * Build a null callback which takes one argument
  * and potentially return a value.
@@ -468,6 +469,7 @@ Callback<R,T1> MakeNullCallback (void) {
 }
 /**
  * \ingroup MakeCallback
+ * \overload Callback<R> MakeNullCallback (void)
  * \return a wrapper Callback
  * Build a null callback which takes two arguments
  * and potentially return a value.
@@ -478,6 +480,7 @@ Callback<R,T1,T2> MakeNullCallback (void) {
 }
 /**
  * \ingroup MakeCallback
+ * \overload Callback<R> MakeNullCallback (void)
  * \return a wrapper Callback
  * Build a null callback which takes three arguments
  * and potentially return a value.
@@ -488,6 +491,7 @@ Callback<R,T1,T2,T3> MakeNullCallback (void) {
 }
 /**
  * \ingroup MakeCallback
+ * \overload Callback<R> MakeNullCallback (void)
  * \return a wrapper Callback
  * Build a null callback which takes four arguments
  * and potentially return a value.
@@ -498,6 +502,7 @@ Callback<R,T1,T2,T3,T4> MakeNullCallback (void) {
 }
 /**
  * \ingroup MakeCallback
+ * \overload Callback<R> MakeNullCallback (void)
  * \return a wrapper Callback
  * Build a null callback which takes five arguments
  * and potentially return a value.

src/core/debug.h

@@ -22,7 +22,7 @@
 #define DEBUG_H
 
 /**
- * \defgroup debugging
+ * \defgroup debugging Debugging
  * \brief Debugging functions and macros
  *
  *   - DEBUG functionality: macros which allow developers to