message: docs: hide GAPI_EXPORTS from documentation
diff: @@ -234,6 +234,8 @@ PREDEFINED = __cplusplus=1 \
  CV_EXPORTS_W_SIMPLE= \
  CV_EXPORTS_AS(x)= \
  CV_EXPORTS_W_MAP= \
  + GAPI_EXPORTS= \
  + GAPI_EXPORTS_W= \
  CV_IN_OUT= \
  CV_OUT= \
  CV_PROP= \
  