Clarify that `upb_Message_SetString` does not copy data.

PiperOrigin-RevId: 613258771

upb/message/accessors.h

@@ -295,6 +295,10 @@ upb_Message_GetString(const upb_Message* msg, const upb_MiniTableField* field,
   return upb_Message_GetField(msg, field, def).str_val;
 }
 
+// Sets the value of a `string` or `bytes` field. The bytes of the value are not
+// copied, so it is the caller's responsibility to ensure that they remain valid
+// for the lifetime of `msg`. That might be done by copying them into the given
+// arena, or by fusing that arena with the arena the bytes live in, for example.
 UPB_API_INLINE bool upb_Message_SetString(upb_Message* msg,
                                           const upb_MiniTableField* field,
                                           upb_StringView value, upb_Arena* a) {