Document using Option<Backtrace> more prominently in docs

[casey]

I missed the change a few versions ago where a Backtrace field is now unconditional, and an Option<Backtrace> only captures a backtrace if RUST_BACKTRACE or RUST_LIB_BACKTRACE is set.

I was a bit confused that backtraces were always being captured, read the docs, and eventually read that I should use Option<Backtrace> in the changelog notes.

I think this could be documented more prominently in the docs, maybe in the Snafu macro docs.