Improve the action UI name getter functions

Change action_get_ui_name() so it will return the name without anything else.
Use it for mentioning actions in the generated help.

Document that action_prepare_ui_name() can insert the probability of success and
a client specific mnemonic.

See patch #4986

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25738 a0f10bec-cc02-0410-94fc-a9cfff90b4cd