doxygen warning/error: documented empty return type of FFT plan functions

@karl-wette for the first time in months, I've been able to build lalsuite successfully on OpenSuse 15.3. Some of your recent build fixes must have magically resolved the linking issues I've had. 👏

Now I just see this when running make html after the successful build. Doxygen is at 1.9.3 on that machine, which seems to be a bit newer than what I have on Debian (1.9.1).

Making html in doxygen
make[2]: Entering directory '/home/dkeitel/git/lvc/lalsuite/lal/doxygen'
doxygen/Makefile: getting input sources ...
doxygen/Makefile: configuring Doxygen build ...
doxygen/Makefile: building Doxygen documentation ...
/home/dkeitel/git/lvc/lalsuite/lal/lib/fft/ComplexFFT.h:193: warning: documented empty return type of XLALDestroyCOMPLEX8FFTPlan
/home/dkeitel/git/lvc/lalsuite/lal/lib/fft/ComplexFFT.h:329: warning: documented empty return type of XLALDestroyCOMPLEX16FFTPlan
/home/dkeitel/git/lvc/lalsuite/lal/lib/fft/RealFFT.h:307: warning: documented empty return type of XLALDestroyREAL4FFTPlan
/home/dkeitel/git/lvc/lalsuite/lal/lib/fft/RealFFT.h:535: warning: documented empty return type of XLALDestroyREAL8FFTPlan
make[2]: *** [Makefile:972: html-local] Error 1
make[2]: Leaving directory '/home/dkeitel/git/lvc/lalsuite/lal/doxygen'
make[1]: *** [Makefile:637: html-recursive] Error 1
make[1]: Leaving directory '/home/dkeitel/git/lvc/lalsuite/lal'
make: *** [Makefile:420: html-recursive] Error 1