Clarify the TOWER_USER_WORKSPACE_ENABLED context for personal secrets and credentials

[mp-seqera]

In Platform Enterprise v25.2.2, personal secrets and credentials still appear in the menu but are unclickable, creating a confusing user experience. Users may think the feature is broken rather than intentionally unavailable.

After confirming with the engineering team, we need to hide or remove this option whenever the personal workspace is disabled via the TOWER_USER_WORKSPACE_ENABLED configuration.

This ticket is to update our documentation so users understand this behavior.

Note: this parameter was first introduced in v24.2.0.