improve linking between proxy config sections
fix a headline
fix code block

Indent the complete block with three more spaces so that it is aligned
properly inside the surrounding list item. Move placeholder comment
to the right place.
minor #4113 tweaks to the new reverse proxy/load balancer chapter (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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tweaks to the new reverse proxy/load balancer chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

bbc2f11 fix code block
6943da8 fix a headline
12840b1 improve linking between proxy config sections