update musharna's tiering

technically Musharna is now BL4, but because there isn't a BL4, I listed it as NU. as far as legality in NU and PU is concerned, this is accurate for the teambuilder and seems to be the best way to handle it.
Merge pull request #1291 from Treecko/patch-5

update musharna's tiering
Fix Inverse Battle
- Immunities turn into weaknesses (I don't know how I could forget that).
- Tools#getImmunity is now similar to Tools#getEffectiveness. Also add a bit of documentation, since there seems to be a lot of confusion about its usage.
AAA: Fix complex ban on Smeargle
Smeargle is banned with Prankster, rather than with Baton Pass. The later was assumed banned due to a typo in the new OP of its forum post.