Update manual segmentation instructions for clarity and completeness

[gryshko]

Summary

Clarifies the Manually segmenting users documentation by adding the required external reference and explicitly documenting a key prerequisite.

Changes made

• Updated the chapter heading to link directly to:
  https://help.nosto.com/en/articles/2398016-visit-custom-event
• Added explicit guidance that manual segmentation requires a backup segment to be created first in Nosto.
• Replaced the broken footnote-style reference with a direct inline link to the same Help Center article.

Why

This removes ambiguity around setup requirements and helps prevent failed manual segmentation attempts caused by missing backup segment configuration.
Related to https://nostosolutions.atlassian.net/browse/NS-13642

Scope

• Docs-only change.
• File updated:
  apis/js-apis/common-examples/manually-segmenting-users.md

Validation

• Verified the updated markdown content renders the intended direct links and includes the prerequisite statement.