[Docs] - Snapshots can be concurrent, but docs not fully updated

[steve-mushero]

Snapshot docs need additional updating now that concurrent Snapshots are allowed.

Some docs were updated in #59358 but seemed to miss others.

Here in the main snapshot page:

"Only one snapshot process can be started in the cluster at any time. "

But from V7.9 (#56911) this seems incorrect and should be updated.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Snapshot/Restore)