Add a note about the default transport layer in gRPC servers

[cb372]

What this does?

Changes, features, fixes ...

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #885 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #885   +/-   ##
=======================================
  Coverage   86.28%   86.28%           
=======================================
  Files          57       57           
  Lines         824      824           
  Branches        5        5           
=======================================
  Hits          711      711           
  Misses        113      113           

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