Improve documentation for assert_frame|series_equal

[qris]

#21528 improved the documentation for assert_almost_equal, but this is actually an undocumented function (not listed in the Pandas docs) and the same issue seems to apply to both assert_series_equal and assert_frame_equal, which are documented.

Could we apply the same change as #21580 to those methods too, i.e. add the following text:

When comparing two numbers, if the first number has magnitude less than 1e-5, we compare the two numbers directly and check whether they are equivalent within the specified precision. Otherwise, we compare the ratio of the second number to the first number and check whether it is equivalent to 1 within the specified precision.

[WillAyd]

Makes sense - a PR from you would certainly be welcome

[sandepp123]

I want to work on this, as it looks like it is good for first timer.