Hide IsSynchronous property

[martintmk]

Details on the issue fix or feature implementation

The property is only used internally and if someone really needs it, we can unhide it later.

Contributes to #1233 (comment)

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]

Codecov Report

Merging #1494 (d12db92) into main (1358076) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1494   +/-   ##
=======================================
  Coverage   83.64%   83.64%           
=======================================
  Files         268      268           
  Lines        6394     6394           
  Branches     1007     1007           
=======================================
  Hits         5348     5348           
  Misses        837      837           
  Partials      209      209           

Flag	Coverage Δ
linux	83.64% <100.00%> (ø)
macos	83.64% <100.00%> (?)
windows	83.64% <100.00%> (?)

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

Files Changed	Coverage Δ
src/Polly.Core/ResilienceContext.cs	100.00% <100.00%> (ø)