Grimtooth and Sunsword invoke can be paid with magic power

If the invoke timeout hasn't run out, the effect will take 25 pw,
the same as a lvl 5 spell.
If hero doesn't have that much, then the invoke fails.

doc/fixes3-7-0.txt

@@ -1510,6 +1510,7 @@ if a tree and a boulder or statue were at the same location, applying an axe
 	would break the boulder or statue rather than chop the tree
 avoid premapping outside Sokoban map to prevent showing stone glyphs
 Grimtooth is permanently poisoned, protects from poison, and can fling poison
+invoke cost for Grimtooth and Sunsword can be paid with magic power
 cursed magic whistle can teleport you to your pet
 Fire and Frost Brand can be invoked for expert level fireball or cone of cold
 wielding Trollsbane grants hungerless regeneration