DOCS/man/lua: suggest keep_open for nested mp.input calls

Before #17251 keep_open was needed to make nested mp.input calls. Even
after that change it is still better to pass it to not show console
quickly closing and reopening to the user, so recommend it.

Author: guidocella

DOCS/man/lua.rst

@@ -931,6 +931,10 @@ REPL.
     ``keep_open``
         Whether to keep the console open on submit. Defaults to ``false``.
 
+        It is recommended to set this to ``true`` when nesting ``input.get()``
+        or ``input.select()`` calls to avoid showing the console quickly closing
+        and reopening.
+
     ``opened``
         A callback invoked when the console is shown. This can be used to
         present a list of options with ``input.set_log()``.
@@ -1027,6 +1031,10 @@ REPL.
     ``keep_open``
         Whether to keep the console open on submit. Defaults to ``false``.
 
+        It is recommended to set this to ``true`` when nesting ``input.get()``
+        or ``input.select()`` calls to avoid showing the console quickly closing
+        and reopening.
+
     Example:
 
         ::