merge vs. join - unify parameter names
#### Problem description

`pandas.DataFrame.merge` function is conceptually simillar like `pandas.DataFrame.join` function.
(first one one merges on specified columns, second merges on index).

These 2 functions use various parameters to do the same thing:
* `join` function has 2 params:  `lsuffix + rsuffix`
* `merge` function has only 1 param: `suffixes`

We should unify names and usage of these parameter(s).
Let's use first way or second way, and deprecate the other way.

_(Personally, I prefer to have only 1 param `suffixes` as it is shorter and there is 1 param for 1 purpose)._
#### Expected Output

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 7
machine          : AMD64
processor        : Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.25.2
numpy            : 1.16.5
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 19.2.3
setuptools       : 41.4.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.1
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : None
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : 0.14.1
pytables         : None
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None

</details>
