docs: update relative QueryConstraint support for Postgres

[cbaker6]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Update documentation to reflect changes in support for the relative QueryConstraint for Postgres.

Related issue: #n/a

Approach

• Update relevant documentation
• nit documentation, mongoDB -> MongoDB

TODOs before merging

• [x] Requires merge of server PR: https://github.com/parse-community/parse-server/pull/7747
• [x] Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov[bot]]

Codecov Report

Merging #305 (3a2242f) into main (1636285) will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
+ Coverage   83.70%   83.73%   +0.03%     
==========================================
  Files         105      105              
  Lines       11154    11154              
==========================================
+ Hits         9336     9340       +4     
+ Misses       1818     1814       -4     

Impacted Files	Coverage Δ
Sources/ParseSwift/Types/Query+async.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/Query+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Types/Query.swift	92.61% <ø> (ø)
Sources/ParseSwift/Types/QueryConstraint.swift	97.20% <ø> (ø)
Sources/ParseSwift/LiveQuery/ParseLiveQuery.swift	73.52% <0.00%> (+0.59%)	:arrow_up:

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