minor #21128 [HttpClient] Add note about concurrency limits (Seldaek)

This PR was merged into the 6.4 branch.

Discussion
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[HttpClient] Add note about concurrency limits

This is kind of an issue report with a half-assed attempt at fixing it.. `@nicolas`-grekas probably should take a look, update wording if needed, and possibly the doc link is borked because I don't know what I am doing with rst.. BUT anyway this is misleading and should be fixed :)

Commits
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7900a2a Add note about concurrency limits

Author: javiereguiluz

http_client.rst

@@ -1355,6 +1355,10 @@ code waits only when necessary.
     or access to certificate files). To avoid hitting these limits, consider
     processing requests in batches.
 
+    There is however a maximum amount of concurrent connections that can be open
+    per host (6 by default). See
+    :doc:`max_host_connections </configuration/framework.html#max-host-connections>`.
+
 Multiplexing Responses
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