update annotations doc due to 'remove' deprecation

[cldougl]

update https://plot.ly/javascript/text-and-annotations/#styling-and-formatting-annotations to reflect plotlyjs update: https://github.com/plotly/plotly.js/pull/1086

[etpinard]

'add' and 'remove' are no longer deprecated as of https://github.com/plotly/plotly.js/pull/1403

[etpinard]

... maybe showing an example with null in the docs isn't a bad idea regardless.