Originally sourced from: https://github.com/libopencm3/libopencm3/pull/382
fixed some typos from the manual and poor merging/rebaseing,
and one judgment call on using a specific name for a conflicting
bit definition.
All the macro arguments that are user supplied, or potentially, wrap properly
in () as good practice.
Probably missed one or two, and a lot of them are possibly unnecessary, but
it's straightforward to just do it always.
Fixes github issue #321
