Add a warning message about CI_COMMIT_REF_NAME in include:rules

What does this MR do?

This MR adds an information about the problem stated in: #337633 (comment 684159945)

From the Slack conversation:

include:rules only works with some variables: https://docs.gitlab.com/ee/ci/yaml/index.html#variables-with-include

And CI_COMMIT_REF_NAME is one of them... Normally, CI_COMMIT_REF_NAME returns just the branch name like "main" or "feature". However, when implementing support of that variable, we used the "ref path" instead of the "ref". So it returns a string like refs/heads/main.

Workaround: you can use if: $CI_COMMIT_REF_NAME =~ /main/.

I think we can solve this problem in this issue when we support all pipeline predefined variables.

I decided to open this MR after seeing this last comment: #276515 (comment 703550311)

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