Merge pull request matplotlib#27536 from meeseeksmachine/auto-backport-of-pr-27534-on-v3.8.x

Backport PR matplotlib#27534 on branch v3.8.x (Clarify AxLine Params)

Author: oscargus

lib/matplotlib/lines.py

@@ -1478,9 +1478,10 @@ def __init__(self, xy1, xy2, slope, **kwargs):
             The first set of (x, y) coordinates for the line to pass through.
         xy2 : (float, float) or None
             The second set of (x, y) coordinates for the line to pass through.
-            Either *xy2* or *slope* has to be given.
+            Both *xy2* and *slope* must be passed, but one of them must be None.
         slope : float or None
-            The slope of the line. Either *xy2* or *slope* has to be given.
+            The slope of the line. Both *xy2* and *slope* must be passed, but one of
+            them must be None.
         """
         super().__init__([0, 1], [0, 1], **kwargs)