Document CLI support for per interface sysctls

[robmry]

- What I did

https://github.com/moby/moby/pull/47686 adds driver-opt label com.docker.network.endpoint.sysctls, to support per-interface sysctls.

The extended --network syntax is needed, but wasn't documented.

The quoting needed to set more than one sysctl using docker [create|run] --network driver-opt or docker network connect --driver-opt isn't obvious - so, added tests and examples to the docs.

- How I did it

• Fix TestNetworkConnectWithFlags - because it didn't do anything, and is needed to test the new option.
• Document the v25.0 changes to allow more than one network connection in create/run, with the extended --network syntax.
• Added tests and examples to docs for per-interface sysctl setting.

- How to verify it

New tests added.

- Description for the changelog

Document the use of `driver-opt` in the CLI for setting per-interface sysctls.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.10%. Comparing base (6c70360) to head (f4cef3e).

:exclamation: Current head f4cef3e differs from pull request most recent head 9f7d811. Consider uploading reports for the commit 9f7d811 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4994   +/-   ##
=======================================
  Coverage   61.09%   61.10%           
=======================================
  Files         298      295    -3     
  Lines       20675    20675           
=======================================
+ Hits        12631    12633    +2     
+ Misses       7147     7143    -4     
- Partials      897      899    +2     

[laurazard]

@thaJeztah does this LGTY?