[Docs] Add notes to use Polly.RateLimiting package

[peter-csala]

Details on the issue fix or feature implementation

• The migration document does not mention explicitly that the new rate limiting strategies reside in a different package than the rest of the strategies.
• I've added notes to doc to help people with the migration process.

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

All modified lines are covered by tests ✅

Comparison is base (6ce4ee0) 84.63% compared to head (8ba239b) 84.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1672   +/-   ##
=======================================
  Coverage   84.63%   84.63%           
=======================================
  Files         306      306           
  Lines        6819     6819           
  Branches     1045     1045           
=======================================
  Hits         5771     5771           
  Misses        839      839           
  Partials      209      209           

Flag	Coverage Δ
linux	?
macos	84.63% <ø> (ø)
windows	84.63% <ø> (ø)

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