Remove misleading information about the initiation of the session server

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What does this MR do?

Through a support request there was a mistake found in the GitLab Runner documentation about the initiation of session for features like the interactive web terminal.

The mistake found was that a session, such as that used by an interactive web terminal, is initiated from GitLab to the GitLab Runner. Therefore, the IP and port configured on the GitLab Runner must be accessible from GitLab and not from the client (web browser).

The current documentation contradicts itself there and leads to the fact that an administrator only checks whether the advertise_address of the session can be reached from his own client, but not from GitLab. As I have tested myself, this advertise_address must only be accessible from GitLab, the web browser or client does not need a direct connection to the GitLab runner and port.

Related issues

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