ICU-22808 Correct a factually incorrect doc comment.

[DavidBarts]

Update doc comment so that the actual behaviour of u_fgetcx is correctly described.

https://unicode-org.atlassian.net/browse/ICU-22808

Checklist

• [ x ] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-NNNNN
  • For minor changes you can use one of the following per-release tickets:
  • ICU 77 code warnings/version updates: ICU-22920 — Fix compiler warnings. Update versions of code-related dependencies (e.g., dependabot).
  • ICU 77 docs minor fixes: ICU-22921 — User Guide & API docs typos etc., and version updates (e.g., dependabot for User Guide)
• [ x ] Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [ x ] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [ x ] Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable