Document the usage of SSLKEYLOGGER

Basically explain https://github.com/grafana/k6/issues/1043 with k6.

Add notice to the `http-debug` that this is an alternative.

https://support.f5.com/csp/article/K50557518 explains how to do it for not k6, but it's more or less the same.

Start by reading k6 issue 1043 and the F5 SSLKEYLOGGER article to understand the behavior to document. Find the k6 documentation for http-debug, explain how SSLKEYLOGGER can be used with k6, and add a notice identifying it as an alternative; done means both topics are clearly covered in the relevant documentation.

Written by the indexing model from the issue text.