DOC: var and std missing examples

[AnshuTrivedi]

• I have checked that the issue still exists on the latest versions of the docs on master here

Location of the documentation

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.var.html
https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.std.html

Documentation problem

In all other APIs in DataFrame at least one example is given for better understanding.
I believe, function description is also available in notebooks but not any example.
It's good to have one example for mentioned APIs.

Suggested fix for documentation

Fix: Use of atleast one example for better undertanding of function.
It's better because function description along with example creates clarity of its use and implimentation.

[attack68]

Sure, fancy contributing a PR?

[MarcoGorelli]

std is done in #44226, so there's still space for a PR for var