docs: clarify weekday range behavior and week start day

[vincent067]

Clarify that Alertmanager follows Go's time.Weekday convention where Sunday = 0 and Saturday = 6. This differs from ISO 8601 where Monday is the first day of the week.

Also explain that weekday ranges do not wrap around the week boundary, so saturday:sunday will not match any day and should be written as ['saturday', 'sunday'] instead.

Fixes #3213

Summary by CodeRabbit

• Documentation
  • Updated weekday_range specification documentation with clarification on weekday numbering system (Sunday=0, Saturday=6), explanation of non-wrapping range behavior across week boundaries, and practical guidance for configuring day combinations correctly.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 423d1c5 and 3517ec5.

📒 Files selected for processing (1)

• docs/configuration.md

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📝 Walkthrough

Walkthrough

Documentation updated to clarify Alertmanager's weekday numbering scheme, explaining that it uses Go's time.Weekday convention (Sunday=0) rather than ISO 8601, and that weekday ranges do not wrap across week boundaries.

Changes

Cohort / File(s)	Summary
Documentation Updates docs/configuration.md	Added clarification text explaining Go time.Weekday numbering (Sunday=0, Saturday=6) for weekday_range specifications, noting that ranges do not wrap (e.g., saturday:sunday matches nothing) and providing guidance to use separate days or sunday:saturday for matching all days.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the documentation update about weekday range behavior and the week start day convention.
Description check	✅ Passed	The PR description clearly explains the changes, references the linked issue, and matches the template requirements for a documentation update.
Linked Issues check	✅ Passed	The PR directly addresses issue #3213 by documenting Alertmanager's use of Go's time.Weekday convention and clarifying that weekday ranges do not wrap.
Out of Scope Changes check	✅ Passed	The change is a documentation-only update directly addressing the requirements in issue #3213 with no out-of-scope code modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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