docs: Import documentation from source-code inline comments.

The inline docs (prefixed with /// in .c files) have been converted to
RST format and put in the docs subdirectory.

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+Power control
+=============
+
+:meth:`pyb.wfi` is used to reduce power consumption while waiting for an
+event such as an interrupt.  You would use it in the following situation::
+
+    while True:
+        do_some_processing()
+        pyb.wfi()
+
+Control the frequency using :meth:`pyb.freq`::
+
+    pyb.freq(30000000) # set CPU frequency to 30MHz