JSON.OBJKEYS: Update description and return values

[LiorKogan]

Clarify behavior of key and path arguments in JSON documentation.

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Note

Low Risk
Low risk documentation-only change clarifying JSON.OBJKEYS argument and return behavior; no runtime code is modified.

Overview
Updates the JSON.OBJKEYS docs to clarify that the command returns object key names for all matches of a given path expression, with explicit distinction between JSONPath vs legacy path syntax (including default path behavior).

Expands the return-value specification for both RESP2/RESP3 to describe differences in errors vs nil vs empty arrays across JSONPath/legacy modes, and adds a note distinguishing Redis keys from JSON object keys plus new related links (JSONPath, JSON.OBJLEN).

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[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

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^{Security scan by Jit}

[dwdougherty]

Thank you, @LiorKogan! I tidied up the return values section per the commit comment (above). Approved. I'll go ahead and merge.