DOC: Fix Dataframe.apply documentation to include note regarding calling function twice on first element

[alonme]

Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.apply.html

Documentation problem

In commit f625730 the documentation of df.apply was changed, and a note regarding df.apply behavior to call the applied function twice on the first column/row.

As far as i can tell from this issue and from my own testing (See below) this behavior still exists.

In [1]:  import pandas as pd

In [2]: df = pd.DataFrame({'a': [1,2,3]})

In [3]: def mul2(x):
    ...:     print('Hello')
    ...:     return x*2
    ...:

In [4]: df.apply(mul2)
Hello
Hello
Out[4]:
   a
0  2
1  4
2  6

In [5]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 41.2.0
Cython           : None
pytest           : 5.3.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : 0.3.3
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : 0.13.1
pyarrow          : None
pytables         : None
pytest           : 5.3.2
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.12
tables           : None
tabulate         : None
xarray           : 0.15.1
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : 0.47.0

Suggested fix for documentation

Revert commit f625730 in order to reflect the actual behavior of the apply function