docs/library/machine: Add docs for Counter and Encoder.

Add documentation for `machine.Counter` and `machine.Encoder` as currently
implemented by the esp32 port, but intended to be implemented by other
ports.

Originally authored by: Ihor Nehrutsa <Ihor.Nehrutsa@gmail.com> and
Jonathan Hogg <me@jonathanhogg.com>.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/reference/glossary.rst

@@ -188,6 +188,13 @@ Glossary
         Most MicroPython boards make a REPL available over a UART, and this is
         typically accessible on a host PC via USB.
 
+    small integer
+        MicroPython optimises the internal representation of integers such that
+        "small" values do not take up space on the heap, and calculations with
+        them do not require heap allocation. On most 32-bit ports, this
+        corresponds to values in the interval ``-2**30 <= x < 2**30``, but this
+        should be considered an implementation detail and not relied upon.
+
     stream
         Also known as a "file-like object". A Python object which provides
         sequential read-write access to the underlying data. A stream object