docs: Update bgpd docs, inc. on decision process, and with a section on MED.

docs: Update bgpd docs, inc. on decision process, and with a section on MED.

* bgpd.texi: Document the -l argument. Update the 'BGP decision process' table
  to reflect what /actually/ is implemented. Add docs on 'compare-routerid' in
  the bestpath section.

  Add a section on MED, to highlight the issues it has by default, and to
  highlight that it is terminally broken for its original purpose in many
  modern iBGP topologies.

  Mention the potential workarounds and fixes.

* routemap.texi: set an anchor on 'set metric' so bgpd.texi can reference it.

(cherry picked from commit d5062d218994885710fe02f516f0c06025b4fc9a)