[Doc] Ack cumulative is not possible in key_shared subscriptions

[AlvaroStream]

Search before asking

• [X] I searched in the issues and found nothing similar.

What issue do you find in Pulsar docs?

we shouldn't ack cumulatively on the shared or key-shared subscription.

What is your suggestion?

Add it

Any reference?

No response

Are you willing to submit a PR?

• [X] I'm willing to submit a PR!