docs: improve documentation sections

[cbaker6]

New Pull Request Checklist

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

Issue Description

Sections in the documentation can be combined for easier reading.

Playgrounds doesn't have an example of unsetting a property.

Related issue: #n/a

Approach

Combine similar sections.

TODOs before merging

• Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant]

Thanks for opening this pull request!

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

[codecov]

Codecov Report

Merging #276 (8ce8905) into main (37a2247) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #276   +/-   ##
=======================================
  Coverage   82.23%   82.23%           
=======================================
  Files         104      104           
  Lines       10574    10574           
=======================================
  Hits         8696     8696           
  Misses       1878     1878           

Impacted Files	Coverage Δ
...cation/3rd Party/ParseApple/ParseApple+async.swift	100.00% <ø> (ø)
...tion/3rd Party/ParseApple/ParseApple+combine.swift	100.00% <ø> (ø)
.../3rd Party/ParseFacebook/ParseFacebook+async.swift	100.00% <ø> (ø)
...rd Party/ParseFacebook/ParseFacebook+combine.swift	100.00% <ø> (ø)
...tication/3rd Party/ParseLDAP/ParseLDAP+async.swift	100.00% <ø> (ø)
...cation/3rd Party/ParseLDAP/ParseLDAP+combine.swift	100.00% <ø> (ø)
...on/3rd Party/ParseTwitter/ParseTwitter+async.swift	100.00% <ø> (ø)
.../3rd Party/ParseTwitter/ParseTwitter+combine.swift	100.00% <ø> (ø)
...Authentication/Internal/ParseAnonymous+async.swift	100.00% <ø> (ø)
...thentication/Internal/ParseAnonymous+combine.swift	100.00% <ø> (ø)
... and 19 more

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