SWIFT-776 Add transaction retry examples

[patrickfreed]

SWIFT-776

This PR adds examples of retrying transactions to the transactions guide. It does so by simply linking to the official docs, since those examples are quite thorough (and will contain the Swift examples written in #478 ).

This PR also adds clarification in the documentation about how options (e.g. write concern) are propagated in transactions. This is based off of feedback from the latest jepsen analysis of MongoDB's transactions.

[codecov-commenter]

Codecov Report

Merging #479 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   76.96%   76.85%   -0.12%     
==========================================
  Files         125      125              
  Lines       12991    12991              
==========================================
- Hits         9999     9984      -15     
- Misses       2992     3007      +15     

Impacted Files	Coverage Δ
Sources/MongoSwift/ClientSession.swift	39.30% <ø> (ø)
.../MongoSwift/Operations/StartSessionOperation.swift	95.45% <ø> (ø)
Sources/MongoSwiftSync/ClientSession.swift	53.57% <ø> (ø)
Tests/MongoSwiftSyncTests/CrudTests.swift	90.90% <0.00%> (-5.69%)	⬇️
Sources/MongoSwift/MongoCollection+Write.swift	97.89% <0.00%> (ø)
Sources/MongoSwift/MongoCollection+BulkWrite.swift	85.62% <0.00%> (ø)

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