DOC: Confusing 'resolvers' kwarg documentation for DataFrame.query()
#### Location of the documentation
According to DataFrame.query() documentation
([https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.query.html#pandas.DataFrame.query](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.query.html#pandas.DataFrame.query))
the 'resolvers' keyword is allowed and its function documented by DataFrame.eval() ([https://pandas.pydata.org/docs/dev/reference/api/pandas.eval.html#pandas.eval](https://pandas.pydata.org/docs/dev/reference/api/pandas.eval.html#pandas.eval)). 

#### Documentation problem
The keyword's description states:
"A list of objects implementing the __getitem__ special method that you can use to *inject an additional* collection of namespaces to use for variable lookup. [...]"

But default namespaces used by DataFrame.eval() are actually overwritten by namespace collections. See basic example:
```
from pandas import DataFrame, Series
import numpy as np
df = DataFrame(np.random.random((5, 3)),)
print(df.query('index==1'))
print(df.query('map==1', resolvers=({'map': Series([4,3,2,1,0], [0,1,2,3,4])},)))
print(df.query('index==1', resolvers=({'map': Series([4,3,2,1,0], [0,1,2,3,4])},)))
```
The last line raises an UndefinedVariableError, because 'index' is no longer part of the namespace collection. 

This is caused by the condition `if resolvers is None:` in DataFrame.eval() source code (v1.0.4, frame.py, line 3335):
```
...
inplace = validate_bool_kwarg(inplace, "inplace")
        resolvers = kwargs.pop("resolvers", None)
        kwargs["level"] = kwargs.pop("level", 0) + 1
        if resolvers is None:
            index_resolvers = self._get_index_resolvers()
            column_resolvers = self._get_cleaned_column_resolvers()
            resolvers = column_resolvers, index_resolvers
        if "target" not in kwargs:
            kwargs["target"] = self
        kwargs["resolvers"] = kwargs.get("resolvers", ()) + tuple(resolvers)
...
```

#### Suggested fix for documentation

Therefore, according to the actual behaviour, the documentation for the 'resolvers' kwarg of DataFrame.eval() should read: 
"A list of objects implementing the __getitem__ special method that you can use to *replace the default* collection of namespaces to use for variable lookup. [...]"

**OR**

Is this a bug? Should it be addressed?
