mention the limit in number of parsers for alt
@@ -17,7 +17,9 @@ pub trait Alt<I, O, E> {
/// Tests a list of parsers one by one until one succeeds.
///
-/// It takes as argument a tuple of parsers.
+/// It takes as argument a tuple of parsers. There is a maximum of 21
+/// parsers. If you need more, it is possible to nest them in other `alt` calls,
+/// like this: `alt(parser_a, alt(parser_b, parser_c))`
///
/// ```rust
/// # #[macro_use] extern crate nom;
