Document TransactionEvent and breadcrumb-related classes.

[st0012]

This is the final part of #1636

[codecov-commenter]

Codecov Report

Merging #1676 (d09daee) into master (95b7db1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1676      +/-   ##
==========================================
+ Coverage   98.46%   98.47%   +0.01%     
==========================================
  Files         136      136              
  Lines        7663     7667       +4     
==========================================
+ Hits         7545     7550       +5     
+ Misses        118      117       -1     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry-ruby.rb	94.28% <ø> (ø)
sentry-ruby/lib/sentry/backtrace.rb	96.61% <ø> (ø)
sentry-ruby/lib/sentry/breadcrumb_buffer.rb	100.00% <ø> (ø)
sentry-ruby/lib/sentry/transaction_event.rb	100.00% <ø> (ø)
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <100.00%> (+4.34%)	⬆️

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