SILBuilder: add comments to constructors.

Author: atrick

include/swift/SIL/SILBuilder.h

@@ -172,14 +172,18 @@ class SILBuilder {
   /// Build instructions before the given insertion point, inheriting the debug
   /// location.
   ///
-  /// Clients should prefer this constructor.
+  /// SILBuilderContext must outlive this SILBuilder instance.
   SILBuilder(SILInstruction *I, const SILDebugScope *DS, SILBuilderContext &C)
       : TempContext(C.getModule()), C(C), F(I->getFunction()) {
     assert(DS && "instruction has no debug scope");
     setCurrentDebugScope(DS);
     setInsertionPoint(I);
   }
 
+  /// Build instructions before the given insertion point, inheriting the debug
+  /// location.
+  ///
+  /// SILBuilderContext must outlive this SILBuilder instance.
   SILBuilder(SILBasicBlock *BB, const SILDebugScope *DS, SILBuilderContext &C)
       : TempContext(C.getModule()), C(C), F(BB->getParent()) {
     assert(DS && "block has no debug scope");