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

Before 17551b2d43 keep_open was needed to make nested mp.input calls.
Even after that change it is still better to pass it to not show the
console quickly closing and reopening to the user, so recommend it.

Co-authored-by: CogentRedTester <cogent.redtester@outlook.com>

DOCS/man/lua.rst

@@ -929,7 +929,12 @@ REPL.
         the text in the console.
 
     ``keep_open``
-        Whether to keep the console open on submit. Defaults to ``false``.
+        Whether to keep the console open on submit, allowing further input.
+        Defaults to ``false``.
+
+        If calling ``input.get()`` or ``input.select()`` again from inside the
+        ``submit`` callback, setting this option to ``true`` allows a seamless
+        transition without the console closing and reopening.
 
     ``opened``
         A callback invoked when the console is shown. This can be used to
@@ -1028,7 +1033,12 @@ REPL.
         the 1-based index of the selected item.
 
     ``keep_open``
-        Whether to keep the console open on submit. Defaults to ``false``.
+        Whether to keep the console open on submit, allowing further input.
+        Defaults to ``false``.
+
+        If calling ``input.get()`` or ``input.select()`` again from inside the
+        ``submit`` callback, setting this option to ``true`` allows a seamless
+        transition without the console closing and reopening.
 
     Example: