Add info about explicit background mode conflicting with tt start

[p7nov]

Resolves #4079
Related to #4014

• Add a note that manual switch to background is prohibited on the tt start page
• Add a note with link to tt start page to box.cfg.background reference
• Add process.* config options reference with minimal (draft) descriptions
• Add a note with link to tt start page to process.background config option reference

Deployment: https://docs.d.tarantool.io/en/doc/gh-4079-tt-start-background/reference/tooling/tt_cli/start/