Clarify that multi-tenancy is meant for the search

[dureuill]

Hello,

As Meilisearch's security model is that there is no multi-tenancy on the "configuration" side of a Meilisearch instance (in particular, webhooks), it should be made explicit:

1. In https://www.meilisearch.com/docs/learn/security/multitenancy_tenant_tokens, add a paragraph with a warning "Multi-tenancy in Meilisearch is supported for users of the search via tenant tokens. Multi-tenancy is not supported for configuring Meilisearch at the time"
2. In https://www.meilisearch.com/docs/reference/api/webhooks, add a paragraph with a warning "Webhooks do not have fine permission granularity, so any API key with webhooks.ACTION permission can accomplish ACTION on all webhooks, regardless of which API key created the webhook.
3. Similarly for https://www.meilisearch.com/docs/reference/api/chats

Acknowledgment: Thanks to the private report of Gabriel Rodrigues, aka Texugo, who prompted the need for clarification