docs: Add quickref and docs for mimxrt, including network.LAN docs.

docs/library/machine.PWM.rst

@@ -78,6 +78,13 @@ Methods
 
    With a single *value* argument the pulse width is set to that value.
 
+Specific PWM class implementations
+----------------------------------
+
+The following concrete class(es) implement enhancements to the PWM class.
+
+   | :ref:`pyb.Timer for PyBoard <pyb.Timer>`  
+
 Limitations of PWM
 ------------------
 
@@ -90,6 +97,11 @@ Limitations of PWM
   80000000 / 267 = 299625.5 Hz, not 300kHz.  If the divider is set to 266 then
   the PWM frequency will be 80000000 / 266 = 300751.9 Hz, but again not 300kHz.
 
+  Some ports like the RP2040 one use a fractional divider, which allow a finer
+  granularity of the frequency at higher frequencies by switching the PWM
+  pulse duration between two adjacent values, such that the resulting average
+  frequency is more close to the intended one, at the cost of spectral purity. 
+
 * The duty cycle has the same discrete nature and its absolute accuracy is not
   achievable.  On most hardware platforms the duty will be applied at the next
   frequency period.  Therefore, you should wait more than "1/frequency" before