Mention null mode in docs for mode option
Mention null mode in docs for mode option

Closes #6034

wingify/CodeMirror,nightwing/CodeMirror,dperetti/CodeMirror,pabloferz/CodeMirror,codio/CodeMirror,pabloferz/CodeMirror,pabloferz/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror,codio/CodeMirror,codio/CodeMirror,dperetti/CodeMirror,dperetti/CodeMirror,hackmdio/CodeMirror,hackmdio/CodeMirror,wingify/CodeMirror,hackmdio/CodeMirror,nightwing/CodeMirror