Document commandine --showing-suggestion

doc_src/cmds/commandline.rst

@@ -113,6 +113,10 @@ The following options output metadata about the commandline state:
     If it is, it would be executed when the ``execute`` bind function is called.
     If the commandline is incomplete, return 2, if erroneus, return 1.
 
+**--showing-suggestion**
+    Evaluates to true (i.e. returns 0) when the shell is currently showing an automatic history completion/suggestion, available to be consumed via one of the `forward-` bindings.
+    For example, can be used to determine if moving the cursor to the right when already at the end of the line would have no effect or if it would cause a completion to be accepted (note that `forward-char-passive` does this automatically).
+
 Example
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