PyHelp tag for spells

[joseasoler]

lfgr coded a new PyHelp for spells, which is included in MNAI. The following ExtraModMod/EitB spells should make use of this new tag to improve their usability. A lot of the vanilla spells could also use improvements in this regard, but that's something that should be tackled in MNAI.

Sever soul - Call form

Extort

Fertility

Rift, create

Rift, open

Rift, cross