Add documentation entry for watch store unsubscription on component

[PonyLux]

What problem does this feature solve?

Actually there are no information regarding the necessity to "unsubscribe" store watch on component destroy.

It would be good to set a warning on the API documentation (https://vuex.vuejs.org/en/api.html) to let users know that without explicit unsubscription, the watch will be keep even after component destroy.

This issue has been reported here #599 and closed as a won't fix.

This problem cause memory leak and in most of the case errors extremely hard to debug.

What does the proposed API look like?

For a minimalist resolution

A warning quote in https://vuex.vuejs.org/en/api.html .

Nice to have

A new entry in menu to explain how to use watch (and then the tips for the unsubscription).
It could be great to give a better knowledge of this feature.

[karanisverma]

@LinusBorg I would like to pick this up

[kiaking]

It could be nice to have this added in the doc I think. It has been awhile but do you want to work on with this @karanisverma?

[karanisverma]

Hey @kiaking, sorry I would not be able to pick this up.

[kiaking]

@karanisverma OK! Thanks for checking 👍

[anschwa]

I would also like to see this get added to the docs. I opened a PR here: #1838