[NFC] Add some notes to getASTType()'s docstring.

include/swift/SIL/SILType.h

@@ -159,6 +159,19 @@ class SILType {
   }
 
   /// Returns the canonical AST type referenced by this SIL type.
+  ///
+  /// NOTE:
+  /// 1. The returned AST type may not be a proper formal type.
+  ///    For example, it may contain a SILFunctionType instead of a
+  ///    FunctionType.
+  /// 2. The returned type may not be the same as the original
+  ///    unlowered type that produced this SILType (even after
+  ///    canonicalization). If you need it, you must pass it separately.
+  ///    For example, `AnyObject.Type` may get lowered to
+  ///    `$@thick AnyObject.Type`, for which the AST type will be
+  ///    `@thick AnyObject.Type`.
+  ///    More generally, you cannot recover a formal type from
+  ///    a lowered type. See docs/SIL.rst for more details.
   CanType getASTType() const {
     return CanType(value.getPointer());
   }