DOC: Improve what the axis= kwarg does for generic methods

[hongshaoyang]

axis=0 or axis=1, which is it?

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 acrossaxis=0" makes it clear that you're summing across indexes (i.e. column sums).

• Closes Improve docs on what the axis= kwarg does in individual functions/methods #29203
• Passes pytest
• Passes black pandas
• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[arw2019]

Thanks @hongshaoyang for the PR!

I agree this is worth improving. However, I think that a better preposition would be along or over rather than across. See, for example, the numpy docs for np.sum:
https://numpy.org/doc/stable/reference/generated/numpy.sum.html#numpy.sum

I'm -1 on this as is

[hongshaoyang]

Thank you @arw2019 for the review! I agree that "over" is clearer, and have changed accordingly.

[hongshaoyang]

Thanks for the approval! May I get this PR labelled as hacktoberfest-accepted?

See #36837 and here.

[rhshadrach]

lgtm; I've verified this takes care of all docs that use the phrase "requested axis".

[jreback]

can you merge master (and ping on green), we just moved a lot of this around.

[hongshaoyang]

all checks passing @jreback

[jreback]

thanks @hongshaoyang