UNDER_THE_HOOD.md: add section on debug code.

Author: peterhinch

UNDER_THE_HOOD.md

@@ -265,19 +265,20 @@ are hard to debug. There is code which I believe is for debugging purposes and
 some I have added myself for this purpose. The aim is to ensure that, if an
 error causes a coro to be scheduled when it shouldn't be, an exception is
 thrown. The alternative is weird, hard to diagnose, behaviour.  
-These are the instances.
-
-[pend_throw(false)](https://github.com/peterhinch/micropython-lib/blob/f20d89c6aad9443a696561ca2a01f7ef0c8fb302/uasyncio.core/uasyncio/core.py#L119)  
-[also here](https://github.com/peterhinch/micropython-lib/blob/f20d89c6aad9443a696561ca2a01f7ef0c8fb302/uasyncio.core/uasyncio/core.py#L123)  
+These are the instances:
+[pend_throw(false)](https://github.com/peterhinch/micropython-lib/blob/f20d89c6aad9443a696561ca2a01f7ef0c8fb302/uasyncio.core/uasyncio/core.py#L119)
+also [here](https://github.com/peterhinch/micropython-lib/blob/f20d89c6aad9443a696561ca2a01f7ef0c8fb302/uasyncio.core/uasyncio/core.py#L123).  
 I think the intention here is to throw an exception (exception doesn't inherit
 from Exception) if it is scheduled incorrectly. Correct scheduling coutermands
-this:  
-[here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L97)  
-[and here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L114)  
-The `rdobjmap` and `wrobjmap` dictionary entries are invalidated  
+this
+[here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L97)
+and [here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L114):
+these lines ensures that the exception will not be thrown.  
+The `rdobjmap` and `wrobjmap` dictionary entries are invalidated
 [here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L91)  
-[and here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L101)  
+and [here](https://github.com/peterhinch/micropython-lib/blob/819562312bae807ce0d01aa8ad36a13c22ba9e40/uasyncio/uasyncio/__init__.py#L101).
 This has the same aim: if an attempt is made incorrectly to reschedule them, an
+exception is thrown.
 
 # Modifying uasyncio