babel: docstring improvement

	* ob.el (org-babel-execute-src-block): Document prefix
	argument in docstring.

lisp/ob.el

@@ -323,6 +323,10 @@ Insert the results of execution into the buffer.  Source code
 execution and the collection and formatting of results can be
 controlled through a variety of header arguments.
 
+With prefix argument ARG, force re-execution even if a an
+existing result cached in the buffer would otherwise have been
+returned.
+
 Optionally supply a value for INFO in the form returned by
 `org-babel-get-src-block-info'.