Highlight file passthrough not working with remote rulesets

What does this MR do and why?

This merge request updates pipeline secret detection documentation to highlight the fact a file passthrough only works with a file committed to the same repository when synthesizing a custom configuration.

This is already mentioned before the example given in line 343 but I thought it's important to highlight this given discussions with support team and to make it clear to customers that this is currently not supported.

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.