docs: clarify glob include/exclude semantics

[magrathean-uk]

Summary

• document bare glob whitelist behavior and ! exclusion behavior
• document the same behavior for case-insensitive and file-based globs
• add a regression test for the existing override semantics

Fixes #503

Validation

• cargo test -p rustic_core glob_patterns_use_override_include_exclude_semantics --locked
• cargo fmt --check
• git diff --check