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

This is not something we can do too much about, without help from the
underlying libraries.

Author: encukou

Doc/library/readline.rst

@@ -45,6 +45,10 @@ 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.
+
 .. data:: backend
 
    The name of the underlying Readline library being used, either