gh-134639: Clarify what "canonical" means in os.path.realpath

Improve documentation for os.path.realpath by clearly defining what a
"canonical path" means. The updated documentation now explicitly states
that a canonical path:
- Is an absolute path
- Has all symbolic links resolved
- Is normalized (redundant separators, '.' and '..' components removed)

Also clarify platform-specific behavior:
- On Windows: resolves MS-DOS (8.3) style names and junction points
- On POSIX: roughly equivalent to the system's realpath() function

Closes #134639

Author: ashm-dev

Doc/library/os.path.rst

@@ -403,10 +403,18 @@ the :mod:`glob` module.)
 
 .. function:: realpath(path, *, strict=False)
 
-   Return the canonical path of the specified filename, eliminating any symbolic
-   links encountered in the path (if they are supported by the operating
-   system). On Windows, this function will also resolve MS-DOS (also called 8.3)
-   style names such as ``C:\\PROGRA~1`` to ``C:\\Program Files``.
+   Return the canonical path of the specified filename. A canonical path:
+
+   * Is an absolute path (always starts from the root of the filesystem)
+   * Has all symbolic links resolved (if supported by the operating system)
+   * Is normalized (redundant separators, ``.`` and ``..`` components are removed)
+
+   On Windows, this function will also resolve MS-DOS (also called 8.3)
+   style names such as ``C:\\PROGRA~1`` to ``C:\\Program Files`` and
+   will resolve junction points.
+
+   On POSIX systems, this function is roughly equivalent to the system's
+   ``realpath()`` function.
 
    If a path doesn't exist or a symlink loop is encountered, and *strict* is
    ``True``, :exc:`OSError` is raised. If *strict* is ``False`` these errors