minor doc update for fluid_voice_is_*()
minor doc update for fluid_voice_is_*()

nezticle/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,nezticle/fluidsynth,nezticle/fluidsynth,FluidSynth/fluidsynth,nezticle/fluidsynth,FluidSynth/fluidsynth