docs: mention letter-spacing helper (#23375)
docs: mention letter-spacing helper (#23375)

Fixes that the `letter-spacing` helper function wasn't mentioned in the docs.

Fixes #23297.
crisbeto/material2,crisbeto/material2,angular/components,josephperrott/material2,josephperrott/material2,josephperrott/material2,crisbeto/material2,josephperrott/material2,crisbeto/material2,angular/components,angular/components,josephperrott/material2,angular/components