[Lock] Add documentation for `InMemoryStore`

[Guikingone]

Hi everyone 👋🏻

Recently, I've faced some bugs when it comes to Lock and during tests, I discovered the InMemoryStore (which has been introduced in 5.2) but didn't found any documentation related to it?

Could it be a good idea to add some informations about it?

Thanks again for the feedback and have a great day 🙂

[javiereguiluz]

Closing as fixed in #17097.