Don't cheat Ash-skill-boosted chars out of their crosstraining bonus (#8901).

This function along with player::skill() badly need refactoring, but the
rounding situation is delicate enough (see 8b15fbdb4) that I'm not going
to refactor it right before 0.15.
Don't let Fedhas worshippers crash at will.

To reproduce: use Growth, enter a number one greater than the number
of available adjacent spaces, then press enter.

Also, make an unreachable return use the correct type.
Don't displace Fedhas-walkable plants when taking stairs (#5117)

This could also be changed for submerged monsters (trapdoor spiders and
air elementals), but that is less noticeable.
Add you.corrosion() to clua.
Fix Song of Slaying description (Internet Kraken)

It claimed to only enhance melee; it also helps with ranged
combat!

Also tweak the following sentence for style + wordcount reasons.