docs: make docstrings concise in expr_str, expr_dt, series_str, ... and more

[MarcoGorelli]

See #1974 for an example of how to make the docstring examples more concise

We've still got the overly long examples in several docstrings

[skritsotalakis]

I can start with expr_str and series_str

[dangotbanned]

@MarcoGorelli do you know of any that are outstanding?

[MarcoGorelli]

this is probably done - thanks all for your help here, really appreciate it 🙏