Fix to long-time imperial conquest CB effect localisation bug

Added some inline documentation, as best I can formalize, of exactly
how vassalize_or_take_under_title works as well. Probably should put
these things in the GitHub Wiki.
Fix and clarification of success effect tooltip for Invasion CB
Invasion CBs: battle_warscore_mult compensates for global -16.7%

Invasion-style CBs which gain all occupied territory:
battle_warscore_mult updated to compensate for global
BATTLE_WARSCORE_WORTH change of -16.7% (prevents effective decrease
below vanilla value)
Much more complete success effect tooltip for Viking Invasion CB
Duchy Adventurers consistently mimic mini-invasion CB mechanics

This change merely makes the Duchy Adventurer CB consistent with
the rest of its mechanics, premise, and purpose. In the event of
the adventurer retinue's [rare] success, they gain any settlements
which they managed to occupy. NOTE: Unlike related invasion CB
mechanics, battle_warscore_mult is unaltered for this CB, making
any occupation beyond more than a county in the target duchy
quite unlikely.
Minor CB tooltip cosmetic change in shia_caliphate_rising

Hardly worth a commit. Only seen when suing for peace. Clarifies
only that the Shia Caliph pretender will, on success, usurp the
Shia Caliphate title as well as the target.
Merge pull request #105 from zijistark/alpha

Assorted CB Fixes / Tweaks [consistency-focused]