DOC: add warning to allclose, revise "Notes" in isclose

[OverLordGoldDragon]

See #10161 (latest comments, ~8/13/2023)

1. Add isclose's warning to allclose
2. Revise isclose's "Notes" on atol to emphasize its poorness and better explain its purpose
3. Add revised isclose note to allclose

[mdhaber]

@OverLordGoldDragon if you are interested in continuing with this, I can review it. The merge conflicts would need to be resolved. Would you like to do that, or would you like me to take care of it and push the change to your branch?

[mdhaber]

I took the liberty of resolving the conflicts and pushing the change to your branch.

I made a few adjustments.

• The text "atol is used exclusively for determining whether a non-zero value in a is close to a zero value in b" could be misinterpreted as meaning that atol is only used when there is a non-zero value in a and a zero value in b, so I modified it.
• The statement " If the expected input values are significantly smaller than one, it can result in false positives." seemed redundant given the example, so I removed it.
• The original statement "atol=0 will result in False if either a or b contains a zero." was not perfectly precise: if both a and b are exactly 0, the result is True. In any case, this didn't seem like either a very subtle or particularly notable consequence of the definition, so I removed it.

If the rendered docs look good, I'll merge when CI finishes.