reindex_like(s, method='ffill') is different than reindex_like(s).fillna(method='ffill')
I don't know whether this a bug or a feature, but the behavior is not clear to me after reading the `reindex_like` docs.

There is a note "Please note: this is only applicable to DataFrames/Series with a monotonically increasing/decreasing index." - but what it actually does is to fill "as if" the index after reindexing was sorted. 

#### Sample code

    >>> import pandas as pd
    >>> s1 = pd.Series(['[0, 1)', '[1, 3)', '[3, 4)', '[4, 6)', '[6, inf)'], index=[0, 1, 3, 4, 6], dtype='string')
    >>> s2 = pd.Series(['']*8, index=[6, 2, 5, 0, 4, 7, 1, 3], dtype='string')
    >>>
    >>> s1
    0      [0, 1)
    1      [1, 3)
    3      [3, 4)
    4      [4, 6)
    6    [6, inf)
    dtype: string
    >>> s2
    6    
    2    
    5    
    0    
    4    
    7    
    1    
    3    
    dtype: string
    >>> s1.reindex_like(s2).fillna(method='ffill')
    6    [6, inf)
    2    [6, inf)
    5    [6, inf)
    0      [0, 1)
    4      [4, 6)
    7      [4, 6)
    1      [1, 3)
    3      [3, 4)
    dtype: string
    >>> s1.reindex_like(s2, method='ffill')
    6    [6, inf)
    2      [1, 3)
    5      [4, 6)
    0      [0, 1)
    4      [4, 6)
    7    [6, inf)
    1      [1, 3)
    3      [3, 4)
    dtype: string

I expected the same result with both methods. Should they behave differently?

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : de_DE.cp1252
pandas           : 1.0.3
numpy            : 1.18.4
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0
Cython           : None
pytest           : 5.4.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.2
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None


</details>
