doc: revert unnamed union/struct workaround in favour of known-issues

A workaround used to silence a warning in the doc generation process
which involved tagging a structure with a #define can now be solved
with a cleaner approach which is non-code-invasive.

Backup said change and update documentation on what to do when the
issue is found.

Change-Id: I1ef5224cd1b2df2e57c2ace438dba90ba3fc8528
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>