Add documentation to internal variable validation methods.
Add documentation to internal variable validation methods.

nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,cellml/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,hsorby/libcellml,hsorby/libcellml