Improve docs on not using operator account outside of the client

[pdyraga]

When the operator account is used outside of the client, the client may lose track of pending transactions. For example:

• client submits a public key to the chain,
• in-browser MetaMask submits transaction with the same nonce but higher gas price and it gets mined,
• client assumes its original submit pubkey transaction has been mined and gives up the publication.

It is not clear from the docs the operator account must not be reused. Let's improve them.