CLI: `--emulator` description and examples about docs

[Eve832]

This command should not yet be surfaced in the documentation according to Stanley.

Here is some info from Prithvi for future documentation:

Design link: https://www.notion.so/Emulator-5fa8f3b627ed424f9bbbd87fdbe56948
Ticket: https://github.com/dfinity/dx-triage/issues/27
Documentation on the emulator as we call it, ic-ref
https://github.com/dfinity-lab/ic-ref/tree/master/impl#the-ic-reference-implementation

"As a developer, I want to be able to use the ic-emulator to run my canisters instead of the replica.
The ic-emulator can (currently) provide some added value to developers by not having to calculate cycles, instant updates and less memory and disk pressure. It is also the reference implementation of the public spec and might have a more consistent behaviour regarding cross canister calls."