JSON.OBJKEYS: Update description and return values

[LiorKogan]

Clarify behavior of key and path arguments in JSON documentation.

---

Note

Low Risk
Low risk documentation-only change that updates the described behavior for JSONPath vs legacy paths and related return/error cases.

Overview
Updates the JSON.OBJKEYS documentation to describe the command as returning JSON object key names for all matches of a path expression, and adds a note distinguishing Redis keys from JSON object keys.

Clarifies key/path argument semantics by explicitly defining JSONPath vs legacy path inputs (including defaults and match behavior), and rewrites the RESP2/RESP3 return information to spell out differing null/empty-array/simple-error outcomes for each path type. Also refreshes cross-links (adds JSON.OBJLEN, JSONPath/topic links).

^{Reviewed by Cursor Bugbot for commit 7aef3c3. Bugbot is set up for automated code reviews on this repo. Configure here.}

[github-actions]

Staging links:
https://redis.io/docs/staging/LiorKogan-patch-1/commands/json.objkeys/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}