[docs] Update Import keys cleos How-to

[bobgt]

Applying feedback received from EOSIO engineers during Doc-a-Thon session 2.

Change Description

Change Type

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• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

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• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions