floss: hcidoc: Add SecurityMode3 tag

Security mode 3 is a legacy security mode for classic connection.
Currently it doesn't work on floss, and it is reported that they
don't work well on BlueZ and iOS either.

Bug: 293397988
Test: m -j
Test: Run hcidoc to analyze report 91211217971
Change-Id: I12e25fdd43e4eeba84479beab9965bc0054c4aba