cli: output of 'cockroach connect --help' is overwhelming

[knz]

When running cockroach connect --help there is a lot of output but no instruction given to the user about what they should be doing.

Would welcome:

• under the "Usage" label at top, a few examples with concrete argument values and explanations (This is the Use field of the cobra.Command in the source)
• also mention --single-node in the usage strings

cc @itsbilal @aaron-crl

part of #60632

Jira issue: CRDB-6276
Epic: CRDB-6663