[comment] Add a semantics note to SIL BindMemory

Author: atrick

include/swift/SIL/SILInstruction.h

@@ -5049,6 +5049,8 @@ class ExplicitCopyAddrInst
 ///
 /// %token is an opaque word representing the previously bound types of this
 /// memory region, before binding it to a contiguous region of type $T.
+///
+/// Semantics: writes an unknown value to memory at `%0` of `%1` bytes.
 class BindMemoryInst final : public InstructionBaseWithTrailingOperands<
                                  SILInstructionKind::BindMemoryInst,
                                  BindMemoryInst, SingleValueInstruction> {
@@ -5096,6 +5098,9 @@ class BindMemoryInst final : public InstructionBaseWithTrailingOperands<
 ///
 /// %out_token represents the previously bound types of this memory region,
 /// before binding it to %in_token.
+///
+/// Semantics: writes an unknown value to memory at `%0` of the same size as the
+/// binding operation that produced `%in_token` bytes.
 class RebindMemoryInst final : public SingleValueInstruction {
   FixedOperandList<2> Operands;