Document DST handling behavior comprehensively

[CybotTM]

Summary

DST (Daylight Saving Time) handling is partially implemented and documented, but users need comprehensive guidance on edge cases.

Current State

• ISC cron behavior for spring-forward implemented (PR Don't skip crons when time jumps forward due to DST robfig/cron#541)
• normalizeDSTDay handles midnight not existing
• checkHourDSTSkip runs skipped-hour jobs immediately
• Basic test for Sao Paulo DST

Documentation Gaps

1. Spring Forward (Clock Jumps Ahead)

• Jobs scheduled in skipped hour: ✅ Run immediately (documented in code)
• User documentation: ❌ Not in README or docs/

2. Fall Back (Clock Repeats Hour)

• Jobs scheduled in repeated hour: ❓ Behavior unclear
• Does job run once or twice?
• Which occurrence is used?

3. Timezone-Specific Behavior

• Different regions have different DST rules
• Some regions don't observe DST
• Historical DST changes

4. Schedule Location vs System Location

• WithLocation() affects parsing
• SpecSchedule.Location affects execution
• Interaction is complex

Required Documentation

README Section

## Daylight Saving Time Handling

### Spring Forward (Clocks Jump Ahead)
When DST causes clocks to spring forward, any jobs scheduled to run 
during the skipped hour will run immediately when the clock jumps.

Example: If DST skips 2:00 AM to 3:00 AM, a job scheduled for 2:30 AM
will run at 3:00 AM.

### Fall Back (Clocks Repeat Hour)
[Document actual behavior]

### Timezone Configuration
[Document WithLocation and SpecSchedule.Location interaction]

docs/DST_HANDLING.md

Comprehensive guide with:

• All DST scenarios
• Examples for different timezones
• Testing recommendations
• Known limitations

Acceptance Criteria

• Document spring-forward behavior in README
• Document fall-back behavior (clarify or implement)
• Create docs/DST_HANDLING.md
• Add examples for common DST scenarios
• Test coverage for fall-back scenario
• Document timezone configuration options