Add missing docs for ParseError and Pointer

[cbaker6]

• Add docs for ParseError so it shows in API docs
• Add docs for Pointer so it shows in API docs. Add fetch publisher for Pointer
• Make ParseStorage private
• Prepare for release

[codecov]

Codecov Report

Merging #125 (cabc799) into main (d1d2ea1) will increase coverage by 0.10%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
+ Coverage   81.26%   81.37%   +0.10%     
==========================================
  Files          65       65              
  Lines        5701     5706       +5     
==========================================
+ Hits         4633     4643      +10     
+ Misses       1068     1063       -5     

Impacted Files	Coverage Δ
...urces/ParseSwift/Objects/ParseObject+combine.swift	100.00% <ø> (ø)
Sources/ParseSwift/Protocols/Fetchable.swift	0.00% <0.00%> (ø)
Sources/ParseSwift/Storage/ParseStorage.swift	88.88% <ø> (ø)
Sources/ParseSwift/Types/ParseError.swift	80.00% <ø> (ø)
Sources/ParseSwift/Types/Pointer.swift	80.00% <100.00%> (+14.28%)	⬆️
Sources/ParseSwift/Objects/ParseObject.swift	77.27% <0.00%> (+0.26%)	⬆️

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