Added JsDuck style comments for animation builtin.
Added JsDuck style comments for animation builtin.

This is an example of how we can document the Alloy builtin libraries for the doc site.

bhatfield/alloy,brentonhouse/brentonhouse.alloy,FokkeZB/alloy,jvkops/alloy,xavierlacot/alloy,4talesa/alloy,jvkops/alloy,indera/alloy,4talesa/alloy,bhatfield/alloy,xavierlacot/alloy,FokkeZB/alloy,FokkeZB/alloy,indera/alloy,xavierlacot/alloy,WY08271/alloy,brentonhouse/brentonhouse.alloy,jppope/alloy,NadsonFernando/alloy,WY08271/alloy,mobilehero/adamantium,brentonhouse/brentonhouse.alloy,4talesa/alloy,WY08271/alloy,NadsonFernando/alloy,jppope/alloy,NadsonFernando/alloy,mobilehero/adamantium,mobilehero/adamantium,jvkops/alloy,jppope/alloy,bhatfield/alloy,indera/alloy