http_client: add information about parameter loading

[tietzsg]

• Inform that the order of the parameters is important when httpcon is loaded first

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

[nchaigne]

The behaviour is actually documented:
"The HTTP connection will be defined using default values in modparam's above the definition of the httpcon in the configuration file."

But this can easily be overlooked. I also had trouble understanding why my configuration did not work initially.
It's better to be more explicit as you propose.

[nchaigne]

Typos : "paramters", "cirtificates"