[3.11] gh-47937: Note that Popen attributes are read-only (GH-93070) (#…

…101684)

* Note that Popen attributes aren't meant to be set by users by rewording the text about the attributes.
* Also update some universal_newlines references to mention the modern text parameter name while in the area.

(cherry picked from commit 027adf4)

Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Doc/library/subprocess.rst

@@ -458,7 +458,7 @@ functions.
  - :const:`0` means unbuffered (read and write are one
  system call and can return short)
  - :const:`1` means line buffered
- (only usable if ``universal_newlines=True`` i.e., in a text mode)
+ (only usable if ``text=True`` or ``universal_newlines=True``)
  - any other positive value means use a buffer of approximately that
  size
  - negative bufsize (the default) means the system default of
@@ -849,7 +849,8 @@ Instances of the :class:`Popen` class have the following methods:
  On Windows :meth:`kill` is an alias for :meth:`terminate`.
 
 
-The following attributes are also available:
+The following attributes are also set by the class for you to access.
+Reassigning them to new values is unsupported:
 
 .. attribute:: Popen.args
 
@@ -862,29 +863,29 @@ The following attributes are also available:
 
  If the *stdin* argument was :data:`PIPE`, this attribute is a writeable
  stream object as returned by :func:`open`. If the *encoding* or *errors*
- arguments were specified or the *universal_newlines* argument was ``True``,
- the stream is a text stream, otherwise it is a byte stream. If the *stdin*
- argument was not :data:`PIPE`, this attribute is ``None``.
+ arguments were specified or the *text* or *universal_newlines* argument
+ was ``True``, the stream is a text stream, otherwise it is a byte stream.
+ If the *stdin* argument was not :data:`PIPE`, this attribute is ``None``.
 
 
 .. attribute:: Popen.stdout
 
  If the *stdout* argument was :data:`PIPE`, this attribute is a readable
  stream object as returned by :func:`open`. Reading from the stream provides
  output from the child process. If the *encoding* or *errors* arguments were
- specified or the *universal_newlines* argument was ``True``, the stream is a
- text stream, otherwise it is a byte stream. If the *stdout* argument was not
- :data:`PIPE`, this attribute is ``None``.
+ specified or the *text* or *universal_newlines* argument was ``True``, the
+ stream is a text stream, otherwise it is a byte stream. If the *stdout*
+ argument was not :data:`PIPE`, this attribute is ``None``.
 
 
 .. attribute:: Popen.stderr
 
  If the *stderr* argument was :data:`PIPE`, this attribute is a readable
  stream object as returned by :func:`open`. Reading from the stream provides
  error output from the child process. If the *encoding* or *errors* arguments
- were specified or the *universal_newlines* argument was ``True``, the stream
- is a text stream, otherwise it is a byte stream. If the *stderr* argument was
- not :data:`PIPE`, this attribute is ``None``.
+ were specified or the *text* or *universal_newlines* argument was ``True``, the
+ stream is a text stream, otherwise it is a byte stream. If the *stderr* argument
+ was not :data:`PIPE`, this attribute is ``None``.
 
 .. warning::