Add feature_enabled_by_namespace_ids to the documentation and CLI

What does this MR do and why?

This MR add minor update to analytics instrumentation docs regarding feature_enabled_by_namespace_ids

and adds feature_enabled_by_namespace_ids identifier to CLI

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Generated event:

config/events/asdf.yml
---
description: asdf
internal_events: true
action: asdf
identifiers:
- feature_enabled_by_namespace_ids
- user
....

Generated code example:

# RAILS

include Gitlab::InternalEventsTracking

track_internal_event(
  'asdf',
  feature_enabled_by_namespace_ids: feature_enabled_by_namespace_ids,
  user: user
)

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

run ruby scripts/internal_events/cli.rb and when prompted for identifiers choose the 6th options (includes feature_enabled_by_namespace_ids)

Related to #454761 (closed)