provide detailed content on stateDuration

stateDuration() is an important function. There is limited content related to it. I suggest we add a section that provides more details on how to leverage this function.

The issue identifies stateDuration() as the subject but names no documentation file, section, or test. First locate the existing documentation for this function and review its current coverage. Done means adding a focused section explaining how to use stateDuration() in practice.

Written by the indexing model from the issue text.