docs: clarify full path help text

[fizyxbt]

Summary

• clarify that --full-path searches the full path, not an absolute output path
• describe the default search target as the final path component
• update the README help snapshot to match

Fixes #1686.

Checks

• cargo fmt -- --check
• cargo clippy --all-targets --all-features -- -Dwarnings
• cargo test --locked

[leno23]

Friendly note: #1997 covers the same #1686 help-text updates with a broader pass (README, man, completion, regression test) and is CI-green. Consider closing this in favor of #1997 to reduce maintainer review load.