📙 Add cluster option explanation

[mtrezza]

Link to section:
Please provide a specific link to the section in question. In the docs, you can click on a heading and then copy the relative link from the address bar.

What is the issue?
parse-community/parse-server#6894 (comment)
https://community.parseplatform.org/t/let-us-know-if-youre-using-parse-in-production/155/20

Can you propose a solution?
Add explanation to docs

[ShravanthReddy]

Opened parse-community/parse-server#10459 to address this bounty by clarifying in the generated Parse Server option docs that cluster is CLI-only and has no effect when set on the ParseServer options object.

[jacog2]

Submitted PR #974 to clarify the cluster option in the Parse Server guide: #974