doc: clarify add/removeListener semantics
doc: clarify add/removeListener semantics

Clarify that adding or removing a listener is not idempotent.

PR-URL: https://github.com/joyent/node/pull/8911
Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>

Cherry-picked-from: https://github.com/joyent/node/commit/48536394c954b2e3e41dcaee3778373ea79e2c25

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node