Documation clarification

[nathanhack]

The symbol.size doesn't explicitly say it supports functions with the common element d, index i, and the array data theme found elsewhere.
https://github.com/d3/d3-shape/blob/master/README.md#symbol_size

Is the intent to support it and the documentation just needs to be updated?
or
Is the documation correct and supporting it is a happy coincidence?
(Just trying to make sure I understand which is intended to be supported moving forward).

[mbostock]

It’s more precise to say that the size accessor receives whatever arguments you pass the symbol generator (which is typically d, i, nodes if you’re using it with d3-selection). The same goes for the type accessor. If you want to contribute a clarification to the README as a PR that would be great!

[nathanhack]

Excellent, thanks for the quick response. PR #145