docs: add "sections" info to example bitcoin.conf

Most bitcoin.conf options apply to all three networks,
however some apply only to mainnet unless specified in a section.
As stands, conf file has no indication that sections are now in use
or are in some circumstances mandatory (eg, changing rpcport for testnet.)

Proposed change notifies the reader early which options are affected,
specifically adds those options affected but not already in the example,
adds brief explanation as to what's going on and provides a skeleton template for the sections themselves.

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