docs: clarify backtrace capture behavior in README

[eastspire]

Closes #409

Summary

The README and the corresponding crate-level rustdoc currently state that
a backtrace is captured and printed with the error "if the underlying
error type does not already provide its own". This wording implies that
anyhow always checks the underlying error, but in fact that check is
only performed on nightly Rust when the unstable
error_generic_member_access feature is enabled (see
src/backtrace.rs). On stable, anyhow unconditionally captures a
backtrace.

This rewording:

• Describes the stable behavior first ("a backtrace is captured and
  printed with the error"), removing the misleading conditional.
• Explicitly calls out the nightly-only opt-out: on nightly with
  error_generic_member_access, anyhow additionally detects when the
  underlying error type already provides a backtrace and avoids
  capturing a second one.

Changes

File	Lines
README.md	78-83
src/lib.rs	131-136

Both files contain essentially the same paragraph (the rustdoc is
mirrored from the README), so they are updated together to keep them
in sync.

Test plan

• cargo build
• cargo test (including 26 doc-tests, which exercise the
  updated rustdoc paragraph)
• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings