doc: proofread light mode and proxy server

Context

Following !6697 (merged), which updated the doc for the proxy mode, this MR updates the pages for the light mode and for the proxy server, as two close derivatives of the former.

Among other things, in the light mode page:

• some of the commands indicated in this page no longer work or generate other results or messages than those stated here
• some recommendations are no more necessary or valid due to new features; for instance the recommended option --protocols
• some other changes here are porting improvements that have been done on the proxy page, on similar pieces of text

Among other things, in the proxy server page:

• the proxy server no more clears the cache of requests after 15s, so the example is updated
• include the command to list the supported protocols
• add an explanation on possible failures when switching to a new protocol
• point more precisely (and reliably) to the source file where heuristics are implemented

Manually testing the MR

Check the updated pages and see if the changes are valid and improve the understanding.

Ideally, execute the instructions to see if they work and give the expected results, as stated in the pages.

Checklist

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• Select as Assignee the next person who should take action on that MR