[z2138] Add some spell description

Partial backport

Also add some more comments in the spell target limitation function

(based on commit [12210] - 2c2f22e)

(based on commit [s1765] - 018ca1e)
[z2139] Remove some code from pathfinding

* Remove dead code
* Remove some optimisation which could cause bad paths. Pathfinder::calculate now depends entirely on the caller to decide when to not calculate the path

(based on commit [12213] - 0afaf3d)

(based on commit [s1766] - a756d26)