doc: Fix warnings associated with 'unbalanced grouping commands'

Builds of docs with doxygen 1.8.16 has a number of warnings of the form:
'warning: unbalanced grouping commands'.  Fix those warnings be either
balancing the group command or removing it.

[Backport of a8171db6]

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>