read_html docs inconsistent with behaviour if lxml not installed
#### Code Sample, a copy-pastable example if possible

```python
>>> dfs = pd.read_html('<table><tr><td>1</td></tr></table>')
ImportError: lxml not found, please install it
>>> dfs = pd.read_html('<table><tr><td>1</td></tr></table>', flavor='bs4')
[ 0
0 1]
```
#### Problem description

The documentation explictly states, in the HTML-parsing-gotchas page and the argument docstring that the fall back is to 'bs4+html5lib' if 'lxml' fails.
For me there is no fallback, just an ImportError since I do not have lxml installed.

#### Expected Output

Docs should be changed to require explicit `flavor` input, or the ImportError is caught.

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

<details>
pandas == 0.25.3
</details>
