Clarify documentation of offsetbox.AnchoredText's prop kw argument

`prop` should be a dictionary of keyword arguments that will be passed to the
`Text` object contained in `AnchoredText` (via `TextArea`). The current
documentation suggests that it should instead be a `FontProperties` object.

This commit changes the documentation to reflect the current behavior.

closes matplotlib#12467

Author: galenlynch

lib/matplotlib/offsetbox.py

@@ -1235,8 +1235,9 @@ def __init__(self, s, loc, pad=0.4, borderpad=0.5, prop=None, **kwargs):
         borderpad : float, optional
             Pad between the frame and the axes (or *bbox_to_anchor*).
 
-        prop : `matplotlib.font_manager.FontProperties`
-            Font properties.
+        prop : dictionary, optional, default: None
+            Dictionary of keyword parameters to be passed to the
+            `~matplotlib.text.Text` instance contained inside AnchoredText.
 
         Notes
         -----