added documentation for default timezone usage

[Faizan711]

Description:

This change adds documentation for the usage of default timezone when data is requested for all sites.

Review

• [x] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [x] Developer changelog updated if needed
• [x] Documentation added if needed
• [x] Existing documentation updated if needed