docs: clarify debug logs when version check or telemetry is disabled

[DmitriyLewen]

Description

When --skip-version-check is used without --disable-telemetry, the request to check.trivy.dev is still sent to deliver anonymous telemetry, but the debug log misleadingly says Running version check.
This confused a user in discussion #10971 into thinking the flag does not work.

This PR makes the debug log messages accurately reflect the actual flag combination:

Flags	Before	After
both	Skipping update check and metric ping	Version check and telemetry are disabled, skipping request
--skip-version-check only	Running version check	Version check is disabled, sending anonymous telemetry only
--disable-telemetry only	Running version check	Telemetry is disabled, running version check only
none	Running version check	Running version check and sending anonymous telemetry
on success with --skip-version-check	Version check completed latest_version=...	Telemetry sent
on success otherwise	Version check completed latest_version=...	Version check completed latest_version=... (unchanged)

The docs are also updated: the Check for updates and Telemetry sections now cross-reference each other and note that both features share a single request to check.trivy.dev, linking to the Check updates service section of the air-gap guide.

Before

$ trivy config -d --skip-version-check .
...
DEBUG  [notification] Running version check

After

$ trivy config -d --skip-version-check .
...
DEBUG  [notification] Version check is disabled, sending anonymous telemetry only

Related issues

• Discussion: --skip-version-check appears not to work #10971

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[mustafakapasi]

lgtm