Add note about Guzzle throwing exceptions

[JaZo]

I had an issue because Guzzle was throwing exceptions for 4xx and 5xx responses and this violates PSR-18. Via Google I stumbled upon php-http/guzzle6-adapter#61 (comment) and found the cause, so the issue is fixed. However, nothing about this is mentioned in the docs, while using a custom Guzzle instance is. This PR adds the comment of php-http/guzzle6-adapter#69 to the documentation so you don't have to Google when the guzzle-adapter is suddenly violating PSR-18.

[dbu]

good point. i think we have that warning in some place already (maybe the phpdoc of the code this example is using) but this looks a likely place where people should be pointed to that fact.

[xabbuh]

indeed, we added it to the docblock in php-http/guzzle6-adapter#69