doc: fix doxygen warnings

doc: fix doxygen warnings

Apparently __<name>__ is not supported for @defgroup commands anymore in
doxygen v1.8.19 (and possibly earlier versions). This results in the
following warning (printed multiple times):

gpiod.h:79: warning: group strong: ignoring title ">high_level</strong> High-level
API" that does not match old title ">common</strong> Common helper macros"

Also the module list is broken.

Fix this my removing the underscores.

Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>