Resolve "[ISC-support #22101] Class priority is underdocumented in the ARM"

Part of #2750 (closed).

• 306eb4cc add makefile rule for ascii umls

  • also adds tkey.atxt and update.atxt sequence diagrams in ASCII art, although not actively used anywhere yet
  • also adds a uml rule to make all targets

• e1a87223 expand on class priority in the ARM

  • There were two places were priorities were discussed so they were conflated in a single section.
  • The distinction between options on one side and lease lifetimes and DHCPv4 fields on the other side was made with regards to priority between classes.

• 26840d4c convert KB diagram into ascii

  The atxt are meant to be used in KB.

#huge-sorry because of regenerating SVGs