Document Transaction and Span classes.

[st0012]

This is a part of #1636.

[codecov-commenter]

Codecov Report

Merging #1653 (e1c57c3) into master (83597d2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1653      +/-   ##
==========================================
+ Coverage   98.47%   98.48%   +0.01%     
==========================================
  Files         136      136              
  Lines        7659     7671      +12     
==========================================
+ Hits         7542     7555      +13     
+ Misses        117      116       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/span.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/transaction.rb	100.00% <100.00%> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <0.00%> (+4.34%)	⬆️

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