minor #14786 [HttpClient] Clarification on use of base_uri + absolute paths (silverbackdan)

This PR was submitted for the 5.x branch but it was squashed and merged into the 4.4 branch instead.

Discussion
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[HttpClient] Clarification on use of base_uri + absolute paths

Related to symfony/symfony#39724

I've tried to word the note as best I can, but I thought it'd be nice to mention the importance and expected behaviour with trailing slashes on the base URI and prefixed slashed on the request path.

Commits
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c295662 [HttpClient] Clarification on use of base_uri + absolute paths

Author: javiereguiluz

http_client.rst

@@ -333,6 +333,10 @@ If you use for example
 as the type and name of an argument, autowiring will inject the ``my_api.client``
 service into your autowired classes.
 
+.. note::
+
+    Pay attention to the trailing slashes in the `base_uri` parameter and prefixed slashes in your path when making a request. The URI will be merged following the rules explained in the [RFC 3986](https://www.ietf.org/rfc/rfc3986.txt) standard. [You can view some examples here](https://symfony.com/doc/current/reference/configuration/framework.html#base-uri)
+
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