Add note for GH importer that secondary email needs to be confirmed

What does this MR do and why?

This MR addresses a comment from https://gitlab.com/gitlab-org/gitlab/-/issues/385438#note_1209407984 and updates our GitHub importer docs by adding a clarifying note that a user's secondary email address requires confirmation. Otherwise user associations may not work when using their email address during an import.

Related to a discussion in #324067 (comment 1210400790)

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