pythongh-140971: Document pathlib.PurePath("") meaning and bool value (python#157293)

pathlib.PurePath() and pathlib.PurePath('') both mean the
current working directory and both have boolean value True.

Patch implements suggestion in
python#140993 (comment).
---------

Co-authored-by: LiuQhahah <liuqiang9596@gmail.com>

Author: terryjreedy

Doc/library/pathlib.rst

@@ -131,10 +131,11 @@ we also call *flavours*:
       >>> PurePath(Path('foo'), Path('bar'))
       PurePosixPath('foo/bar')
 
-   When *pathsegments* is empty, the current directory is assumed::
+   When *pathsegments* is empty or consists only of empty strings,
+   the current directory is assumed::
 
-      >>> PurePath()
-      PurePosixPath('.')
+      >>> PurePath(), PurePath('')
+      (PurePosixPath('.'), PurePosixPath('.'))
 
    If a segment is an absolute path, all previous segments are ignored
    (like :func:`os.path.join`)::
@@ -1040,15 +1041,17 @@ Querying file type and status
 
 .. method:: Path.exists(*, follow_symlinks=True)
 
-   Return ``True`` if the path points to an existing file or directory.
-   ``False`` will be returned if the path is invalid, inaccessible or missing.
+   Return ``True`` if the path points to an existing file or directory and
+   ``False`` if the path is invalid, inaccessible or missing.
    Use :meth:`Path.stat` to distinguish between these cases.
 
    This method normally follows symlinks; to check if a symlink exists, add
    the argument ``follow_symlinks=False``.
 
    ::
 
+      >>> Path('').exists()  # The current directory.
+      True
       >>> Path('.').exists()
       True
       >>> Path('setup.py').exists()