add is_literal docs entry

py-polars/docs/source/reference/expressions/meta.rst

@@ -13,6 +13,7 @@ The following methods are available under the `expr.meta` attribute.
     Expr.meta.has_multiple_outputs
     Expr.meta.is_column
     Expr.meta.is_column_selection
+    Expr.meta.is_literal
     Expr.meta.is_regex_projection
     Expr.meta.ne
     Expr.meta.output_name