[MIRROR] small modlink fixes [MDB IGNORE] (#23293)

* small modlink fixes (#77804)

## About The Pull Request
fixes bug in multitools where they didnt unregister signals
fixes modlink scryers checking battery charge without a battery
frequency stuff is a bit more explained

## Why It's Good For The Game
good stuf

## Changelog
🆑
fix: fixes multitools possibly randomly losing their buffer
fix: fixes modlinks checking battery charge without a battery and
working without charge
qol: modlink stuff is a bit more explained
fix: modlinks printed from rnd no longer start with a frequency
(remember to copy it from another one with a multitool buffer, robotics
starts with 2 NT frequency linked ones)
/🆑

* small modlink fixes

---------

Co-authored-by: Fikou <23585223+Fikou@users.noreply.github.com>

code/game/objects/items/devices/multitool.dm

@@ -42,8 +42,8 @@
 	return OXYLOSS//theres a reason it wasn't recommended by doctors
 
 /obj/item/multitool/proc/set_buffer(datum/buffer)
-	if(buffer)
-		UnregisterSignal(buffer, COMSIG_QDELETING)
+	if(src.buffer)
+		UnregisterSignal(src.buffer, COMSIG_QDELETING)
 	if(QDELETED(buffer))
 		return
 	src.buffer = buffer