Update clients reference

[lampajr]

Changes proposed

Update the clients reference documentation based on the following api changes:

• Make inner http client configurable horreum-client-golang#15
• Make inner http client configurable horreum-client-python#15

They are going to change how the HorreumClient is initialized

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.

[lampajr]

I changed the way the HorreumClient needs to be instatiated (in terms of params) because some additional configuration have been required (see linked pull requests for more details)