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.