Add an explanation why the CI variable value cannot be masked

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What does this MR do and why?

1. Add an explanation why the CI variable value cannot be masked in the error message.
2. Include a link in the Masked help text that points to the Mask a CI/CD variable documentation.

Screenshots or screen recordings

Before	After
before.mov	after.mp4

How to set up and validate locally

1. Navigate to the group/project CI/CD settings.
2. Expand Variables section.
3. Click an Add variable button.
4. In the opened drawer under the Visibility section choose Masked option, enter invalid variable value.
5. Click a link in the help text under Masked option to navigate to the documentation.

Related to #465258 (closed)