[MAINT] Add docstrings to spectral time methods

[adam2392]

PR Description

Towards: #127

@ruuskas if you have the time, would you be able to review the docstrings here to verify that they are correct?

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [ ] whats_new.rst is updated
• [ ] PR description includes phrase "closes <#issue-number>"

[adam2392]

Perhaps @tsbinns you are interested in giving this a review since you're interested in spectral connectivity?