Update HAL API header information

hal/critical_section_api.h

@@ -33,8 +33,11 @@ extern "C" {
   * This function is called directly by core_util_critical_section_enter on
   * first entrance to a critical section.
   *
-  * The default behavior of this function is to save the current state of
-  * interrupts before disabling them.
+  * There is a default implementation of this function which will save the
+  * current state of interrupts before disabling them. This implementation can
+  * be found in mbed_critical_section_api.c. This behaviour is can be overridden
+  * on a per platform basis by providing a different implementation within the
+  * correct targets directory.
   *
   * The function is only called once per critical section by
   * core_util_critical_section_enter. When implementing this function for a
@@ -49,8 +52,11 @@ void hal_critical_section_enter(void);
   * This function is called directly by core_util_critical_section_exit on the
   * final exit from a critical section.
   *
-  * The default behavior of this function is to restore the state of interrupts
-  * as they were prior to entering this critical section.
+  * There is a default implementation of this function, it will restore the
+  * state of the interrupts as they were prior to entering this critical
+  * section, this implementation can be found in mbed_critical_section_api.c.
+  * This behavior is overridable by providing a different function
+  * implementation within the correct targets directory.
   *
   * This function is only called once per critical section. When implemented
   * for a specific platform it must restore any state that was saved upon

platform/mbed_critical.c

@@ -61,10 +61,10 @@ void core_util_critical_section_enter(void)
 // FIXME
 #ifdef FEATURE_UVISOR
     #warning "core_util_critical_section_enter needs fixing to work from unprivileged code"
-#endif /* FEATURE_UVISOR */
-
+#else
     // If the reentrancy counter overflows something has gone badly wrong.
     MBED_ASSERT(critical_section_reentrancy_counter < UINT32_MAX);
+#endif /* FEATURE_UVISOR */
 
     if (critical_section_reentrancy_counter == 0) {
         hal_critical_section_enter();