Is there are any documentation about user-defined menu in midnight-commander?
It is documented in the help, the node is "Edit Menu File" under "Command Menu"; if you scroll down you should find "Addition Conditions":
If the condition begins with '+' (or '+?') instead of '=' (or '=?') it is an addition condition. If the condition is true the menu entry will be included in the menu. If the condition is false the menu entry will not be included in the menu.
This is preceded by "Default conditions" (the = condition), which determine which entry will be highlighted as the default choice when the menu appears. Anyway, by way of example:
+ t r & ! t t
t r means if this is a regular file ("t(ype) r"), and ! t t means if the file has not been tagged in the interface.
- Midnight Commander
man
On top what has been written above, this page can be browsed in the Internet, when searching for man pages, e.g.: https://www.systutorials.com/docs/linux/man/1-mc/
Search for "Menu File Edit".
- Here you could find another good description: http://www.softpanorama.org/OFM/MC/user_menu.shtml
Best regards, Jarek