`value_label` is not documented

#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc)
Ubuntu 19.10
Firefox 73.0 (64-bit)
python 3.7
bokeh 1.4.0
holoviews 1.12.7
holoviz 0.11.3
hvplot 0.5.2
jupyter 6.0.0
jupyterlab 1.2.6
jupyterlab_server 1.0.6
dask 2.10.1

#### Description of expected behavior and the observed behavior
Clicking in jupyterlab on the `line` part of `my_df.hvplot.line` and using `shift+enter` a small popup with available args for the method `line` opens. However the `value_label` arg is not documented.