Improve configurability documentation

[gregestren]

Spawned from https://github.com/bazelbuild/bazel/issues/5574#issuecomment-667299335.

Specific ideas:

• [ ] Update configurable build attributes intro doc to reflect Starlark flag support
• [ ] Minimize uses of --define in doc examples
• [ ] Platforms example: why is everything specified twice? (from https://github.com/bazelbuild/bazel/issues/5574#issuecomment-667670328)
• [ ] Make the difference between constraint_setting, config_setting, build_setting, platform, etc. clearer
• [ ] State current reality and best practices vs. "this will be possible in the future"
• [ ] Clearer, more actionable error messaging

[gregestren]

Also review reference docs:

• [ ] https://docs.bazel.build/versions/master/be/functions.html#select
• [ ] https://docs.bazel.build/versions/master/be/common-definitions.html#configurable-attributes
• [ ] https://docs.bazel.build/versions/master/be/general.html#config_setting

[gregestren]

Also:

• [ ] clarify difference between cfg = "exec" and cfg = "host"

[gregestren]

Also: beautify error messages.