DOC: Undocumented change in .isin behavior from 1.1.5 to 1.2.0
#### Location of the documentation
https://pandas.pydata.org/docs/whatsnew/v1.2.0.html

#### Documentation problem
There seems to be an undocumented change with how `isin` works.
pandas 1.1.5:
```python
import pandas as pd
pd.Series([0]).isin(['0']) 
# 0    True
# dtype: bool
pd.Series([1]).isin(['1']) 
# 0    True
# dtype: bool
pd.Series([1.1]).isin(['1.1']) 
# 0    True
# dtype: bool
```
But in pandas 1.2.0 (and 1.2.0rc):
```python
import pandas as pd
pd.Series([0]).isin(['0']) 
# 0    False
# dtype: bool
pd.Series([1]).isin(['1']) 
# 0    False
# dtype: bool
pd.Series([1.1]).isin(['1.1']) 
# 0    False
# dtype: bool
```

But the release notes mentions nothing about it.
I have not posted this issue as an bug,  because I think the newer implementation is the better implementation.
However I propose adding the change to the release notes for other people like me where the change broke tests in a pipeline.

#### Suggested fix for documentation

Add a line mentioning that `isin` no longer returns true when comparing floats to a string of the float.
