docs(list): clarify that md-avatar-icon is for use with icons (#11396)
docs(list): clarify that md-avatar-icon is for use with icons (#11396)

current example causes confusion that it can be used with images
it also uses an icon set that is not defined in the docs site

Fixes #10382
bluebirdtech/material,miguelcobain/material,bluebirdtech/material,oostmeijer/material,marekmicek/material,marekmicek/material,Splaktar/material,oostmeijer/material,miguelcobain/material,oostmeijer/material,Splaktar/material,clshortfuse/material,clshortfuse/material,marekmicek/material,miguelcobain/material,Splaktar/material,angular/material,bluebirdtech/material,bluebirdtech/material,angular/material,angular/material,oostmeijer/material,angular/material,marekmicek/material,miguelcobain/material,Splaktar/material,clshortfuse/material,clshortfuse/material