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Clarify image-related services docs (#10815)

...and make one of the comments accurate by fixing the code. Turns
out I forgot to add the 'silent' argument when I changed that comment.
I
Ian Hickson committed
f4e4b93c1fd9d27fc0340e24c6d82ecb9a76731d
Parent: ba520e7
Committed by GitHub <noreply@github.com> on 6/23/2017, 9:06:30 PM