Documentation: Conditional Compilation section needs improvement.

[WeeBull]

The section "Options for Conditional Compilation" mainly consists of a code example, with very little explaination. In particular it's not obvious what the difference between (very similarly named) std.Build.option() and std.Build.addOptions() is and why both are needed. The documentation comments in the standard library don't really help either.

Best explanation I found was this blog post.