Issue 798 - Auditing PFMERGE command docs

[Kannav02]

This PR aims to find and fix inconsistencies in the documentation for the PFMERGE command as mentioned in the issue #798

The objectives of this PR can be simplified in the points below

• [x] Run the command and its examples in a local instance of DiceDB and make sure they work as per requirements
• [x] Compare the command with Redis's counterpart and make sure that the results are the same
• [x] In the official documentation, format the headings with the correct title as specified in the issue
• [x] Remove the Conclusion heading if it exists
• [x] Appropriate use of headers
• [x] Convert Parameters and Return Value to a markdown table format to ensure consistency

end goal - documentation should match the format followed by SET and be consistent with how the code works

Fixes #798