Add example of using pair.punct_mut()

src/punctuated.rs

@@ -947,6 +947,22 @@ impl<T, P> Pair<T, P> {
 
     /// Mutably borrows the punctuation from this punctuated pair, unless the
     /// pair is the final one and there is no trailing punctuation.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use proc_macro2::Span;
+    /// # use syn::punctuated::Punctuated;
+    /// # use syn::{parse_quote, Token, TypeParamBound};
+    /// #
+    /// # let mut punctuated = Punctuated::<TypeParamBound, Token![+]>::new();
+    /// # let span = Span::call_site();
+    /// #
+    /// punctuated.insert(0, parse_quote!('lifetime));
+    /// if let Some(punct) = punctuated.pairs_mut().next().unwrap().punct_mut() {
+    ///     punct.span = span;
+    /// }
+    /// ```
     pub fn punct_mut(&mut self) -> Option<&mut P> {
         match self {
             Pair::Punctuated(_, p) => Some(p),