Document right dropdown alignment property

What does this MR do?

The right prop of gl-dropdown already existed as a pass-through option to b-dropdown, but it was not obvious that this could be utilized as it is not documented explicitly in our storybook page and in the pajamas examples. Adding this should boost the visibility of this option a bit.

Does this MR meet the acceptance criteria?

Conformity

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• Added the ~"component:*" label(s) if applicable.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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closes #999 (closed)