Draw attention to Position::Only in docs

Author: kornelski

src/lib.rs

@@ -1948,7 +1948,7 @@ pub trait Itertools: Iterator {
     }
 
     /// Return an iterator adaptor that combines each element with a `Position` to
-    /// ease special-case handling of the first or last elements.
+    /// ease special-case handling of the first, last, or [singular](Position::Only) elements.
     ///
     /// Iterator element type is
     /// [`(Position, Self::Item)`](Position)

src/with_position.rs

@@ -45,14 +45,17 @@ where
 /// The first component of the value yielded by `WithPosition`.
 /// Indicates the position of this element in the iterator results.
 ///
+/// When handling the first or last position,
+/// remember to consider the special case of [`Position::Only`].
+///
 /// See [`.with_position()`](crate::Itertools::with_position) for more information.
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum Position {
-    /// This is the first element.
+    /// This is the first element, and there is more than one element.
     First,
     /// This is neither the first nor the last element.
     Middle,
-    /// This is the last element.
+    /// This is the last element, and there was more than one element.
     Last,
     /// This is the only element.
     Only,