Document permission required for time tracking

What does this MR do?

I recently noticed that a user account wasn't able to use the /spend quick action to log time on an issue. I eventually figured out that was because this user account had the Guest role, whereas apparently the Reporter role (or higher) is required in order to use /spend. I checked the Permissions table but didn't see time tracking mentioned, so this MR attempts to document the current behavior.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because this MR changes only documentation.
• Documentation (if required)
• [ ] Code review guidelines
• [ ] Merge request performance guidelines
• Style guides
• [ ] Database guides
• [ ] Separation of EE specific content

Availability and Testing

No functional changes.

Security

No functional changes.