Documentation issue: FeatureConfiguration

Documentation URL: https://docs.bazel.build/versions/master/skylark/lib/FeatureConfiguration.html

This documentation is missing any kind of detail of what a feature actually is. Also has no context of what CROSSTOOL is, or what a CROSSTOOL feature is.

This should be written to explain what a FeatureConfiguration actually represents rather than referring to implicit knowledge of the historical context of what a CROSSTOOL file was and what was defined in it.

Start with the FeatureConfiguration documentation at the linked Documentation URL and identify the existing explanation and references to CROSSTOOL. Read the surrounding documentation to establish the intended audience and terminology. Done means the page explains what a feature and FeatureConfiguration represent, including the relevant CROSSTOOL context, without relying on historical implicit knowledge.

Written by the indexing model from the issue text.