docs: Add additional example/note for Timer's callback usage.

Add example: using named function for the Timer's callback.
Add note: improving traceback inside interrupt timers.

docs/library/micropython.rst

@@ -29,7 +29,8 @@ Functions
 
    Allocate ``size`` bytes of RAM for the emergency exception buffer (a good
    size is around 100 bytes).  The buffer is used to create exceptions in cases
-   when normal RAM allocation would fail (eg within an interrupt handler).
+   when normal RAM allocation would fail (eg within an interrupt handler) and
+   therefore give useful traceback information in these situations.
 
    A good way to use this function is to put it at the start of your main script
    (eg boot.py or main.py) and then the emergency exception buffer will be active

docs/library/pyb.Timer.rst

@@ -18,6 +18,13 @@ Example usage to toggle an LED at a fixed frequency::
     tim.init(freq=2)                # trigger at 2Hz
     tim.callback(lambda t:pyb.LED(1).toggle())
 
+Example using named function for the callback::
+
+    def tick(timer):                # we will receive the timer object when being called
+        print(timer.counter())      # show current timer's counter value
+    tim = pyb.Timer(4, freq=1)      # create a timer object using timer 4 - trigger at 1Hz
+    tim.callback(tick)              # set the callback to our tick function
+
 Further examples::
 
     tim = pyb.Timer(4, freq=100)    # freq in Hz
@@ -32,6 +39,10 @@ Further examples::
 the servo driver, and Timer 6 is used for timed ADC/DAC reading/writing.
 It is recommended to use the other timers in your programs.
 
+*Note:* Memory can't be allocated during a callback (an interrupt) and so
+exceptions raised within a callback don't give much information.  See
+:func:`micropython.alloc_emergency_exception_buf` for how to get around this
+limitation.
 
 Constructors
 ------------