Clarify U-mode deprecation in open() (GH-11646)

Clarify U-mode deprecation in open() (GH-11646)

The previous wording could be read as saying that universal
newlines mode itself was deprecated, when it's only the 'U'
character in the mode field that should be avoided.

The update also moves the description of the 'U' mode character
out of the mode table, as the longer explanation was overly
intrusive as a table entry and overshadowed the actually useful
mode characters.
(cherry picked from commit 3171df34141c1f26ec16dccb4357184c0cf6c58f)

Co-authored-by: Nick Coghlan <ncoghlan@gmail.com>