Make it clear that examples do not always reflect the default settings

What does this MR do?

This just adds a note on the page calling out that the examples provided in the gitlab.rb do not always reflect the default settings for an instance. For example:

https://docs.gitlab.com/omnibus/settings/configuration.html#disable-the-varoptgitlab-directory-management

This value, once uncommented is not the default:

manage_storage_directories['enable'] = false

But we have it in our gitlab.rb as a commented out value, when the default value is actually true.

The point in this is to have a sentence in our docs that confirms that not all examples provided in the template are default, which some support engineers may not even be aware of. I know I and others were not aware of this until right before I made this MR.

Related issues

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