docs(waiter): add doctests for utility pattern matching functions

# What was changed

- Added doctests for _contains_match showing patterns found, not found, and spanning multiple lines

- Added doctests for _regex_match demonstrating string patterns, compiled patterns, and regex groups

- Added doctests for _match_regex_across_lines showing multi-line pattern matching

- Fixed implementation in _match_regex_across_lines to correctly determine starting line of multi-line matches

- Fixed doctest in _contains_match to properly handle raw string escaping

# Why these changes were made

- Documentation as code: Provides executable examples of these utility functions

- Quality assurance: Verifies that functions behave as expected

- Previous line detection couldn't correctly identify which line a multi-line match started on

- The new position-based approach correctly calculates starting line by counting newlines

- The doctest fixes ensure correct testing of multi-line pattern matching

Author: tony