Backport one line from 54affdb3f37e
Mention rebinding behavior of +=

Explicitly mention "rebind", hopefully facilitates search
Ref: https://groups.google.com/d/msg/julia-users/ujxesTieDHU/zWrZNAUHelkJ
(cherry picked from commit 9c7a490b348fe097209b302a790c4b5482774969)