Merge pull request swiftlang#42498 from aschwaighofer/document_readonly_metadata_create

Document that we intentionally mark functions that create metadata as `readonly` such that they cannot be hoisted above availability checks

Author: aschwaighofer

include/swift/Runtime/RuntimeFunctions.def

@@ -15,6 +15,8 @@
 //
 // Runtime functions that read from object arguments cannot be marked
 // ReadNone. Otherwise the objects may be freed before the runtime call.
+// Runtime functions that instantiate metadata cannot be marked ReadNone
+// or else they could be moved out of blocks controlled by a availability check.
 //
 //===----------------------------------------------------------------------===//