Readme claims backtraces captured "if the underlying error type does not already provide its own"

[jorendorff]

This is true inasmuch as the sentence uses "if" and not "if and only if", but I was misled by the phrasing.

IIUC, anyhow can only check to see if the underlying error provides a backtrace if the unstable error_generic_member_access feature is enabled.

If so, I can do a quick PR to say this in the readme. It's only a sentence.