bpo-41887: omit leading spaces/tabs on ast.literal_eval (#22469)

Also document that eval() does this (the same way).
diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index c49bb0c..263c52a 100644
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -506,6 +506,9 @@
    returns the current global and local dictionary, respectively, which may be
    useful to pass around for use by :func:`eval` or :func:`exec`.
 
+   If the given source is a string, then leading and trailing spaces and tabs
+   are stripped.
+
    See :func:`ast.literal_eval` for a function that can safely evaluate strings
    with expressions containing only literals.