Disable debug print for missing DSR

From investigating a few cases from the logs it looks like the main case
without a DSR is a DOMFragment inside extension or transclusion content.
Document this and disable the debug print, as this case is actually pretty
common.

Change-Id: Ibf826dd72e3576fba8a04fa91e931b8e4d7cae46
Merge "Disable debug print for missing DSR"