Document runtime config

[dogukanteber]

What this PR does: Provides documentation for runtime_config

Which issue(s) this PR fixes: Fixes #5153

Checklist

• [x] Tests updated
• [x] Documentation added
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[dogukanteber]

Linter failed by saying this:

pkg/ingester/instance_limits.go:37:1: receiver-naming: receiver name l should be consistent with previous receiver name cfg for InstanceLimits (revive)
func (l *InstanceLimits) UnmarshalYAML(unmarshal func(interface{}) error) error {
	if defaultInstanceLimits != nil {
		*l = *defaultInstanceLimits
	}
	type plain InstanceLimits // type indirection to make sure we don't go into recursive loop
	return unmarshal((*plain)(l))
}
make: *** [Makefile:183: lint] Error 1

But I did not touch this part of the code. Should I fix it or leave it as is?

[stale[bot]]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.