DOC: Improve what the `axis=` kwarg does for generic methods
### [`axis=0` or `axis=1`, which is it?](https://github.com/pandas-dev/pandas/issues/29203#issue-511923432)
Opening a PR to discuss #29203 on what it means to do `df.sum(axis=0)` versus `df.sum(axis=1)`. The docs does not make it intuitively clear to the user. I propose a simple change where **"summing across`axis=0`"** makes it clear that you're summing across indexes (i.e. **column sums**).


- [x] Closes #29203
- [x] Passes pytest
- [x] Passes `black pandas`
- [x] Passes `git diff upstream/master -u -- "*.py" | flake8 --diff`