docs: improve m.request return value description

[GAumala]

Description

In the m.request return value description, add a line informing that error status codes cause the promise to reject.

Motivation and Context

This info is already available in the "How it works" section, but if I specifically want to know under what conditions m.request rejects, I'd go straight to the signature description.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md