DOC: get_indexer returns non-matching with -1 positional
[An answer on my recent SO post](https://stackoverflow.com/a/65085536/4107349) used `get_indexer`, which worked great, but I soon learned if the indexer doesn't find a match it seems to use the last entry without raising an error:

```
df = pd.DataFrame({"ID": [6, 2, 4],
                   "to ignore": ["foo", "whatever", "idk"],
                   "value": ["A", "B", "asdf"],
                   })

df2 = pd.DataFrame({"ID_number": [1, 2, 3, 4, 5, 6],
                    "A": [0.91, 0.42, 0.85, 0.84, 0.81, 0.88],
                    "B": [0.11, 0.22, 0.45, 0.38, 0.01, 0.18],
                    })

df2 = df2.set_index('ID_number')
df['new_col'] = df2.values[df2.index.get_indexer(df['ID']), df2.columns.get_indexer(df['value'])]
```

I presumed the row with "asdf" would have raised an error, not returned the value for "B". This was problematic because I unwittingly processed data incorrectly and I enjoy being employed.

My interpretation of the [documentation](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Index.get_indexer.html) was that if `method` was not supplied it would be "default: exact matches only". Supplying `method = 'default'` and `tolerance = 0` was also not accepted when not using another method.

Also, I'm no serious programmer so I have likely misunderstood something and am only trying to help. Please feel free to correct / tell me to go away