docs: update addKeyboards for Keyman Engine for Web to clarify async and update examples

[mcdurdin]

the addKeyboards documentation page and examples need updating -- they use some out of date examples and don't mention that the call is async.
https://help.keyman.com/DEVELOPER/engine/web/14.0/reference/core/addKeyboards
https://help.keyman.com/DEVELOPER/engine/web/14.0/guide/adding-keyboards

[ermshiperete]

See also keymanapp/keyman.com#449