allowClear does not show button when multiple = true about ember-power-select

[cibernox]

The allowClear option only works with single selects. I never thought about making it work with multiple selects since with them the usual thing to do is to remove only on element at a time (or use backspace).

I basically copied select2/selectize and they never provided that option, so neither did I.

[corrspt]

I see, from the docs I didn't understand that. (Guess I should make a PR with that added, right :P?)

I guess that's a valid approach. I was thinking when you have several choices and wanted to remove them all, but I guess it's probably not that much of a requirement.

[cibernox]

If you could add a paragraph clarifying that this option only works for single selects that'd be great

[corrspt]

I might have some free time tonight, I'll try to help!

[corrspt]

I created #105 to help with documentation. I also included the information about allow clear in that PR