Document gitignore inputs that raise despite being accepted by Git

[rupayon123]

Document the GitIgnorePatternError boundary on both supported pattern_to_regex methods: a bare ! (including trailing unescaped spaces) and an unmatched trailing backslash raise rather than producing a no-op. Git accepts these lines without a diagnostic, so a caller cannot assume that a Git-readable ignore file is exception-free for pathspec.

This addresses the documentation-only request in #146 and its follow-up, preserving runtime behavior and the existing exception contract. It does not overlap #143's proposed behavior change.

Validation:

• Executed each example against both current parser classes and checked blank/comment controls.
• Verified Git accepts the examples without diagnostics using git check-ignore --no-index in an isolated repository.
• python -m unittest discover -t . -s tests/ succeeds: 221 tests run, 379 skipped cases/subtests for unavailable optional backends/platform conditions.
• Strict Sphinx HTML build (-aWEnqb html) passes; both API sections contain the new text and exception links.
• git diff --check passes. Optional native regex backends were not installed or tested.

Prepared with OpenAI Codex assistance.

[cpburnz]

#143 and #146 have been fixed. In the course of fixing those, I've lightly documented when exceptions are raised in pattern_to_regex().