p2p: review and update the documentation of the p2p configuration parameters

[cason]

The following content needs to be updated accordingly with the implementation:

• Description of the configuration parameters for the configuration file: config/toml.go
• Documentation of configuration parameters on Go source code: config/config.go
• Documentation of configuration parameters, currently in spec/p2p/legacy-docs/config.md, potentially to be moved (back, once updated) to spec/p2p/config.md (intersects with spec/p2p: review, update, and consolidate the legacy documentation #981 )
• Documentation of configuration parameters under docs/ (e.g. docs/core/running-in-production.md, docs/core/validators.md)
• Verify and validate the differences on all this content between main and release branches

As a stretch goal:

• Reduce the degree of redundancy on the documentation
• Render the backporting process simpler by highlighting parts of the documentation and configuration flags that are specific for a given version

[cason]

@greg-szabo, are you considering this as part of #2351?