docs: document benchmark_min_time CLI forms

[felmonon]

Fixes #1878.

This is a docs-only PR that:

• documents the supported --benchmark_min_time forms in docs/user_guide.md
• clarifies that the s suffix is preferred for time values, while bare numeric values are still accepted
• documents the iteration-count form (x) for benchmarks that do not already specify explicit iterations
• adds the required contributor entries described in CONTRIBUTING.md

Tests not run (docs-only change).

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[dmah42]

thank you