Update i18n documentation on pluralization

Current guidelines in GitLab product documentation: https://docs.gitlab.com/ee/development/i18n/externalization.html#plurals

Following the discussions on these two merge requests...:

• gitlab-org/gitlab!178303 (comment 2340948820)
• gitlab-org/gitlab!178639 (comment 2340922787)

..we need to improve the pluralization giddiness.