pythongh-59254: mention in open() doc that line buffering is for writing

Doc/library/functions.rst

@@ -1221,7 +1221,7 @@ are always available.  They are listed here in alphabetical order.
 
    *buffering* is an optional integer used to set the buffering policy.  Pass 0
    to switch buffering off (only allowed in binary mode), 1 to select line
-   buffering (only usable in text mode), and an integer > 1 to indicate the size
+   buffering (only usable when writing in text mode), and an integer > 1 to indicate the size
    in bytes of a fixed-size chunk buffer. Note that specifying a buffer size this
    way applies for binary buffered I/O, but ``TextIOWrapper`` (i.e., files opened
    with ``mode='r+'``) would have another buffering. To disable buffering in