Doc: distributed reads enabled by default

What does this MR do?

As performance issue gitaly!2372 (merged) with database query has been fixed. The feature is switched to be enabled by default, but still remains under control of the feature flag gitaly!2470 (merged). It could be disabled by disabling a feature flag in console.
The testing was done on prod under gitlab-com/gl-infra/production#2508 (closed).

Relates to: gitaly#2951 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content