Prevent using commas in url configurations

Literal commas are not allowed in the url configuration because
they are used as a separator internally. Add document in the
resource to prevent OEMs to misconfigure it and cause unexpected
result.

Bug: 155027030
Test: m -j30; atest NetworkStackTests
Change-Id: I76a847105fc6e57542bb7b8e2dc7ce45ae8393a3