[3.10] gh-47937: Note that Popen attributes are read-only (GH-93070) (#101683)

* 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 027adf42cd85db41fee05b0a40d89ef822876c97)

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