Document setting context of Stache data helper

[marshallswain]

As mentioned in #1619, this documents setting the context of the data helper explicitly.

Does the "optional argument" syntax work in that @Signature?

Closes #1619

[daffl]

Isn't context here more the data that are being set?

[marshallswain]

I might misunderstand how it works. I thought the optional argument was to allow setting another key as an alternative to the current scope. Maybe it should just say key instead of context?

[daffl]

Well I mean basically what you do with <div {{data 'person' person2}}> is $(div).data('person', person2) so it's simply just the value that you are setting. I find the word context potentially confusing here.

[alexisabril]

This was fixed by commit b7c8769