Identically named media fields may cause cropping conflict

[kylewayneluck]

Discovered w/ @ifox earlier this week. If multiple media fields with identical names are rendered in the same template, only one of their cropping specifications will be used. An example would be a block editor image (named "image"), and a browser field image (named "image").

In the short term, we should make a note of this in our docs (perhaps requesting that developers uniquely name any media fields that are rendered on the same page).