amphbomb.lua: removed burrow (cloak when idle) ability, commented some code that cause error.

this limpet don't seem to need a firepoint (gun) vfx, so that commented code is not necessary,
scripts/amphbomb.lua: undo burrow removal and make burrow only when above water, and fix dig-dig special effect when burrowing.
units/amphbomb.lua: add comment on special effects
Merge pull request #2 from xponen/patch-6

amphbomb.lua: removed burrow (cloak when idle) ability, commented some code that cause error.