Merge pull request matplotlib#26200 from oscargus/coloralphaversion

[Doc] Add note about (str, alpha) version added

Author: rcomer

galleries/users_explain/colors/colors.py

@@ -70,6 +70,8 @@
 +--------------------------------------+--------------------------------------+
 | Tuple of one of the above color      | - ``('green', 0.3)``                 |
 | formats and an alpha float.          | - ``('#f00', 0.9)``                  |
+|                                      |                                      |
+| .. versionadded:: 3.8                |                                      |
 +--------------------------------------+--------------------------------------+
 
 .. _xkcd color survey: https://xkcd.com/color/rgb/