Clean up flag restriction documentation

* "Airbase" is now a userflag, so may not exist in all rulesets
* "IgTer" no longer has any hardcoded terrain nativity restrictions

See patch #4852

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25296 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Gen-move support for autoexplorers

See patch #4853

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25297 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Un-hardcode safe house discovery

See patch #4854

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25298 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Allow amphibious unit for kill_something_with()

See patch #4855

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25299 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Remove unused ai channel cache

See patch #4856

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25300 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Use terrain_class rather than move_type in AI

Used to select Attackers, Bodyguards, Hunters, and Settlers

See patch #4858

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25301 a0f10bec-cc02-0410-94fc-a9cfff90b4cd