[3.12] gh-121160: Note that readline libraries using different history formats. (GH-121327) (GH-122031)

This is not something we can do too much about, without help from the
underlying libraries.
(cherry picked from commit 709db44255)

Doc/library/readline.rst

@@ -44,6 +44,10 @@ Readline library in general.
     python:bind -v
     python:bind ^I rl_complete
 
+  Also note that different libraries may use different history file formats.
+  When switching the underlying library, existing history files may become
+  unusable.
+
 
 Init file
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