make note of one more normalization that Paths do

Fixes rust-lang#29008

Author: steveklabnik

src/libstd/path.rs

@@ -2202,6 +2202,8 @@ impl Path {
     ///   beginning of the path. For example, `a/./b`, `a/b/`, `a/b/.` and
     ///   `a/b` all have `a` and `b` as components, but `./a/b` starts with
     ///   an additional [`CurDir`] component.
+    /// 
+    /// * A trailing slash is normalized away, `/a/b` and `/a/b/` are equivalent.
     ///
     /// Note that no other normalization takes place; in particular, `a/c`
     /// and `a/b/../c` are distinct, to account for the possibility that `b`