Update tctl docs to include new global flags and remote functionality.

[Joerger]

This PR updates tctl docs:

• Include the newer --identity, --insecure, and --auth-server global flags.
• Add info about connecting to an auth or proxy server remotesly.
• Adds a note about auditing tctl actions to avoid future issues similar to Add real username/id of user who performed the action instead of host UUID to teleport audit logs #6333

[tcsc]

It's fine as it is, but there are a couple of clarifications that could be made if you are up for another round.

[awly]

Please also update the tctl --help output to match