Fix documentation for AbstractChannel and AbstractOutput, which were inadvertently interleaved. [ci skip]
Fix documentation for AbstractChannel and AbstractOutput, which were inadvertently interleaved. [ci skip]

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core