Add to docs: API v4 response formats for multiple prospects

[alice-mezza]

When using the eg /api/prospect/version/4/do/read/email/ operation with API v4, the response may now contain multiple prospects, instead of just one like in v3. But the documentation about response formats is the same for both v3 and v4:
http://developer.pardot.com/kb/api-version-4/prospects/#xml-response-formats
http://developer.pardot.com/kb/api-version-3/prospects/#xml-response-formats

What the different variations of responses look like for v4 (single or multiple prospects) should be added to the documentation.