docs: Add expression() parser to combinator list

Resolves:
* winnow-rs#804 (comment)

Author: ssmendon

src/combinator/mod.rs

@@ -89,6 +89,8 @@
 //! - [`fail`]: Inversion of [`empty`]. Always fails.
 //! - [`Parser::by_ref`]: Allow moving `&mut impl Parser` into other parsers
 //!
+//! - [`expression()`]: Parse an operator precedence expression with Pratt parsing
+//!
 //! ## Text parsing
 //!
 //! - [`any`][crate::token::any]: Matches one token