docs(rooms): the noticeOf comment says an empty notice is not encoded, but an empty notice clears the guest copy #919

Written-by: Dev3

core/src/host/room-membership.js (noticeOf) says: "an empty value is not encoded, so it never wipes a notice". That is the opposite of what ships: encodeRoomShare sends notice: '' on a join, applyInboundRoomShare applies a present empty string (clearing the guest's copy), and core/test/remote-room.test.js asserts both ("a cleared notice is still carried, as an empty field", "clearing the notice clears the guest's copy").

Scope: correct that one comment to "a present empty value clears the notice, an absent field leaves it"; no behaviour change, one commit, no new test (the behaviour is already covered). Raised by Reviewer2 as non-blocking in the #907 conclusion.