[HttpClient] Added notes about handling errors when multiplexing responses#12896

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a-menshchikov wants to merge 1 commit intosymfony:4.3from

a-menshchikov:http-client-first-chunk

Commits on Jan 17, 2020

Added notes about multiplexing responses that contain ones with 4xx/5xx status codes and about handling timeout in the same case
a-menshchikov
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