known-issues: clarify documentation on ignore blocks

Some regular expressions used to ignore outputs could use some extra
documentation to clarify what was being ignored.

Change-Id: Iabf3ce6964a89cdb7f9bbe8d99a3b4b8f98da59b
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>