Document giterr_last() use only after error. #4772

docs/error-handling.md

@@ -117,6 +117,8 @@ The public error API
   the last error.  This may return NULL if no error has occurred.
   Otherwise this should return a `git_error` object indicating the class
   of error and the error message that was generated by the library.
+  Do not use this method unless you received an error, it will 
+  lead to undefined behaviour.
 
   The last error is stored in thread-local storage when libgit2 is
   compiled with thread support, so you do not have to worry about another