doc: format Scores extension
doc: format Scores extension

why:
* it was confusing to read when all the extension fields were
collapsed into one paragraph
* also point out that submission_data is ignored when content_items
are passed

closes INTEROP-7287
flag=none

test plan:
* view http://canvas.docker/doc/api/score.html
and note that the extension field's description is very crowded
* run `dcr web rake doc:api` after checking this out
* reload the doc page and it should be much easier to parse and
understand

Change-Id: I88c5117d77009764f23629bf493261ccb3668def
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/284888
Reviewed-by: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
Product-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms