Add a note that {$USER.MACRO} can't be used in macro functions

The documentation now states that you can use user macros in item names. It is separately indicated that you can use macro functions with LDD macros
Therefore, it is not obvious that using in the item name a construction like

{{#LLDMACRO}.regsub("{$TEST.REGEXP}",\1}

won't work ass expected
Let's add a note about that