Improve std::path::Path::join documentation

Adapt `PathBuf::push` documentation for `Path::join` and add it to `join`'s docs

Author: KaiTomotake

library/std/src/path.rs

@@ -2971,6 +2971,15 @@ impl Path {
     /// Creates an owned [`PathBuf`] with `path` adjoined to `self`.
     ///
     /// If `path` is absolute, it replaces the current path.
+    /// 
+    /// On Windows:
+    ///
+    /// * if `path` has a root but no prefix (e.g., `\windows`), it
+    ///   replaces and returns everything except for the prefix (if any) of `self`.
+    /// * if `path` has a prefix but no root, `self` is ignored and `path` is returned.
+    /// * if `self` has a verbatim prefix (e.g. `\\?\C:\windows`)
+    ///   and `path` is not empty, the new path is normalized: all references
+    ///   to `.` and `..` are removed.
     ///
     /// See [`PathBuf::push`] for more details on what it means to adjoin a path.
     ///