Update doc to clarify the behavior when removing key/value pairs during iteration #76

Job	Run time
test-linux-amd64 / test-linux (linux-unit-test-1-cpu)	11m 11s
test-linux-amd64 / test-linux (linux-unit-test-2-cpu)	12m 24s
test-linux-amd64-race / test-linux (linux-unit-test-4-cpu-race)	20m 15s
test-linux-amd64 / test-linux (linux-unit-test-4-cpu)	12m 22s
coverage	11m 48s
	1h 8m 0s

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