docs(material/tree): add a description of the when predicate (#21545)
docs(material/tree): add a description of the when predicate (#21545)

- update TODO comments since issue was closed as a duplicate,
  point to the new issue
angular/components,andrewseguin/material2,crisbeto/material2,angular/components,josephperrott/material2,angular/components,mmalerba/material2,DevVersion/material2,andrewseguin/material2,mmalerba/material2,mmalerba/material2,DevVersion/material2,DevVersion/material2,mmalerba/material2,crisbeto/material2,crisbeto/material2,DevVersion/material2,josephperrott/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,josephperrott/material2,mmalerba/material2,andrewseguin/material2,andrewseguin/material2,josephperrott/material2,angular/components,andrewseguin/material2