DOC: update the DataFrame.iat[] docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
# paste output of "scripts/validate_docstrings.py <your-function-or-method>" here
# between the "```" (remove this comment, but keep the "```")

################################################################################
####################### Docstring (pandas.DataFrame.iat) #######################
################################################################################

Selects a single value for a row/column pair by integer position.

Useful if performance is a major concern and you only need to
get or set a value at a particular row/column.

See Also
--------
at : Selects a single value for a row/column label pair
loc : Selects a group of rows and columns by label(s)
iloc : Selects group of rows and columns by integer position(s)

Returns
-------
    None

Examples
--------
>>> df = pd.DataFrame([[0,2,3], [0,4,1], [10,20,30]])
>>> df
    0   1   2
0   0   2   3
1   0   4   1
2  10  20  30
>>> df.iat[1, 2]
1
>>> df.iat[1, 2] = 10
>>> df.iat[1, 2]
10

################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.DataFrame.iat" correct. :)

```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.
Thank you for the comments! Please let me know if there is anything else that should be changed. Thanks for the updates! Thanks @akosel !