go.tools/ssa: doc: Builtin is not an Instruction.
go.tools/ssa: doc: Builtin is not an Instruction.

Fixes issue 7026.

R=gri
CC=golang-codereviews
https://codereview.appspot.com/48630045

karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools,chenjianlong/go.tools,lattera/go.tools,chenjianlong/go.tools,karmakaze/go.tools,lattera/go.tools,karmakaze/go.tools