EXIF/IPTC captions should populate caption/post_excerpt on upload, not description/post_content

[nacin]

It's been this way forever, but it is dumping data into the wrong field. Two schools of thought on this:

• Plugins have used the description field, so we should not change this.

• Users continue to struggle with moving *captions* from the description field to the caption field, so we should change this.

I am in the latter school of thought. A plugin can remain compatible by ensuring it checks either description or caption before falling back to the other.