Document interaction of `namespace` and `open`

[TwoFX]

What question should the reference manual answer?

The current example on open does not contain any cases where open appears inside namespace. The reference manual does not explain that in

def A.B.C.a1 : Nat := 0
def A.C.a2 : Nat := 0
def C.a3 : Nat := 0

namespace A
open B C

#check a1
#check a2
#check a3 -- unknown identifier `a3`

the namespaces _root_.A.B.C and _root_.A.C come into scope, while _root_.C does not.