Add description of useTransactionsInternally to docs

[cbaker6]

• Add missing documentation for ParseSwift configuration
• Test linux on Swift 5.4.1
• Remove extra linux build from CI
• README mention 1.8.1 and up because of internal transaction usage that doesn't work with mongoDB

[codecov]

Codecov Report

Merging #162 (fb71222) into main (973cf33) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
- Coverage   82.10%   82.07%   -0.04%     
==========================================
  Files          70       70              
  Lines        6582     6582              
==========================================
- Hits         5404     5402       -2     
- Misses       1178     1180       +2     

Impacted Files	Coverage Δ
Sources/ParseSwift/Parse.swift	100.00% <ø> (ø)
Sources/ParseSwift/Coding/AnyEncodable.swift	60.52% <0.00%> (-1.32%)	⬇️
Sources/ParseSwift/Storage/ParseFileManager.swift	89.31% <0.00%> (-0.77%)	⬇️
Sources/ParseSwift/Types/Query.swift	93.17% <0.00%> (-0.18%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	77.54% <0.00%> (+0.53%)	⬆️

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