nitg: rename `common_ffi` group to `ffi`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix typo in doc of `MParameterType`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix typo in doc of `before_send`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>