Add documentation for data helper

Resolves issue #574

Added a deprecation notice for the {{ data helper }}

@function can-stache.helpers.data {{data name}}
@parent can-stache/deprecated

@deprecated {4.3.0} Use [can-stache.helpers.domData] instead.

@signature `{{data name[ key]}}`

Adds the current [can.stache.context context] to the
element's [can.data].