Update Prawn::Images#image method documentation to be more precise

Looks like some time between 0.12.0 and 0.15.0 the requirement for this object went from only responding to #read to also needing to respond to #rewind.  Now, if it doesn't respond to #rewind, it tries to treat it as a path and complains that it's not a string.

My pull request simply adds to the documentation that an object also needs to respond to #rewind now.