docs: iron-image position property should define valid values

[Splaktar]

Description

The docs for iron-image's position property define a default value of center but they don't define what other values are valid.

Relevant code:

iron-image/iron-image.html

Line 178 in d0699b5

value: 'center'

Expected outcome

Similar to the docs for iron-image's sizing property it should list the valid values.

Actual outcome

center appears to be the only valid value of this property. Searching the code and repository doesn't turn up anything helpful.

Live Demo

https://elements.polymer-project.org/elements/iron-image#property-position

[davidmaxwaterman]

+1 this sort of thing is quite irritating

[davidmaxwaterman]

For the record, it seems to be a background-position:

https://developer.mozilla.org/en-US/docs/Web/CSS/background-position