Initial preliminary API Review (1)

[martintmk]

Details on the issue fix or feature implementation

Just addressing some of the issues found when doing internal API review. We want to do some initial pass before we engage with broader audience.

• Using defaults for CancellationToken.
• Fixed some docs.
• Hiding IsEmpty from callback API.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov-commenter]

Codecov Report

Merging #1185 (2a98a87) into main (3ff908a) will not change coverage.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##             main    #1185   +/-   ##
=======================================
  Coverage   83.42%   83.42%           
=======================================
  Files         273      273           
  Lines        6329     6329           
  Branches     1028     1028           
=======================================
  Hits         5280     5280           
  Misses        844      844           
  Partials      205      205           

Flag	Coverage Δ
macos	83.42% <100.00%> (ø)
windows	83.42% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...eaker/BaseCircuitBreakerStrategyOptions.TResult.cs	100.00% <ø> (ø)
...ircuitBreaker/BaseCircuitBreakerStrategyOptions.cs	100.00% <ø> (ø)
...Core/CircuitBreaker/CircuitBreakerManualControl.cs	100.00% <ø> (ø)
...y.Core/Fallback/FallbackStrategyOptions.TResult.cs	100.00% <ø> (ø)
src/Polly.Core/Hedging/HedgingHandler.TResult.cs	100.00% <ø> (ø)
src/Polly.Core/Hedging/VoidHedgingHandler.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategy.Async.Task.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategy.Async.TaskT.cs	100.00% <ø> (ø)
src/Polly.Core/ResilienceStrategy.SyncT.cs	100.00% <ø> (ø)
src/Polly.Core/Strategy/Outcome.TResult.cs	100.00% <ø> (ø)
... and 14 more