Six and a half years later, and this thread is still the only way to find this rule...  Seems like something that would be useful in the glossary or the Search results...

This is a monster ability and the MM has a very clear section about it. Not sure where else you need to put the rule tbh.

Limited Usage

Some special abilities have restrictions on the number of times they can be used.

X/Day. The notation “X/Day” means a special ability can be used X number of times and that a monster must finish a long rest to regain expended uses. For example, “1/Day” means a special ability can be used once and that the monster must finish a long rest to use it again.

Recharge X–Y. The notation “Recharge X–Y” means a monster can use a special ability once and that the ability then has a random chance of recharging during each subsequent round of combat. At the start of each of the monster’s turns, roll a d6. If the roll is one of the numbers in the recharge notation, the monster regains the use of the special ability. The ability also recharges when the monster finishes a short or long rest.

For example, “Recharge 5–6” means a monster can use the special ability once. Then, at the start of the monster’s turn, it regains the use of that ability if it rolls a 5 or 6 on a d6.

Recharge after a Short or Long Rest. This notation means that a monster can use a special ability once and then must finish a short or long rest to use it again.