[SPARKR][MINOR] Add more examples to window function docs

## What changes were proposed in this pull request?

This PR adds more examples to window function docs to make them more accessible to the users.

It also fixes default value issues for `lag` and `lead`.

## How was this patch tested?

Manual test, R unit test.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14779 from junyangq/SPARKR-FixWindowFunctionDocs.