Update http client doc

[FabioNappi]

Updated http client docs so that it is clear that the http client forwards Mia headers only if it is created from the request

Checklist

• your branch will not cause merge conflict with master
• run npm test
• tests are included
• the documentation is updated or integrated accordingly with your changes
• the code will follow the lint rules and style of the project
• you are not committing extraneous files or sensible data

[coveralls]

Coverage: 95.063%. Remained the same when pulling e73a3cb on update/http-client-doc into 26eb47d on master.

[coveralls]

Coverage: 95.063%. Remained the same when pulling e73a3cb on update/http-client-doc into 26eb47d on master.