Rename tolerance to distinguish from time in doc

[Fujikawas]

't' is used for both tolerance and time in these equations, which can be confusing.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[IshaanDesai]

Rather than saying tol, it would be better to use a mathematical symbol. It would be worth looking at what the typical mathematical symbols are for tolerances.