docs: update selection toggle to conform with point selection behavior

[arvind]

When single and multi selections were merged into a point selection, it no longer became possible to disabling toggling to yield only multi-point insertion (as was possible with the old multi selection).

Partially addresses #8066.

📦 Published PR as canary version: 5.3.1--canary.8270.ef8c0b6.0

:sparkles: Test out this PR locally via:

npm install [email protected]
# or 
yarn add [email protected]

[vega-org-bot]

:rocket: PR was released in v5.6.0-next.1 :rocket: