SWIFT-853 Clarify requirements for when when cursors/change streams need to be killed

[patrickfreed]

SWIFT-853

[codecov-commenter]

Codecov Report

Merging #488 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #488   +/-   ##
=======================================
  Coverage   76.89%   76.89%           
=======================================
  Files         125      125           
  Lines       13012    13012           
=======================================
  Hits        10005    10005           
  Misses       3007     3007           

Impacted Files	Coverage Δ
Sources/MongoSwift/ChangeStream.swift	30.64% <ø> (ø)
Sources/MongoSwift/MongoClient.swift	87.09% <ø> (ø)
...ces/MongoSwift/MongoCollection+ChangeStreams.swift	43.75% <ø> (ø)
Sources/MongoSwift/MongoCollection+Indexes.swift	89.33% <ø> (ø)
Sources/MongoSwift/MongoCollection+Read.swift	100.00% <ø> (ø)
Sources/MongoSwift/MongoCursor.swift	100.00% <ø> (ø)
Sources/MongoSwift/MongoDatabase.swift	80.35% <ø> (ø)
Sources/MongoSwiftSync/ChangeStream.swift	0.00% <ø> (ø)
Sources/MongoSwiftSync/MongoCursor.swift	90.62% <ø> (ø)

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[nbbeeken]

docs look nicely formatted in xcode's doc popups 🚀

[kmahar]

lgtm!