Polish "Pluggable HTTP client" section in reference docs

[izeye]

Description

This PR polishes the "Pluggable HTTP client" section in the reference docs a bit.

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments as appropriate. Consider including links in comments to relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (77f7f81) 79.88% compared to head (d5974e9) 79.88%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1644   +/-   ##
=========================================
  Coverage     79.88%   79.88%           
  Complexity     2195     2195           
=========================================
  Files           209      209           
  Lines          6667     6667           
  Branches        364      364           
=========================================
  Hits           5326     5326           
  Misses         1127     1127           
  Partials        214      214           

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