Clarify old sample validation flag scope

[Rohithmatham12]

What this changes

• Clarifies that validation.reject-old-samples and validation.reject-old-samples.max-age are enforced by the distributor before samples are forwarded.
• Clarifies that late samples rejected by ingester TSDB because they are older than the head minimum time are controlled by ingester.out-of-order-time-window.
• Updates both the flag help text and configuration docs so operators see the same guidance in CLI and docs.

Fixes #6220.

Testing

• GOCACHE=/private/tmp/cortex-go-cache GOMODCACHE=/private/tmp/cortex-go-mod-cache go test ./pkg/util/validation -run 'TestLimits_Validate|TestLimits_ValidateQueryLimits' -count=1
• git diff --check

Note: a broader local go test ./pkg/util/validation run hit an unrelated segmentation fault in the existing TestValidateNativeHistogram path before this change was involved.