Add a clarification about variable precedence for child pipelines

What does this MR do and why?

In #387183 (comment 1244605923), I've discovered that when in a downstream pipeline, inherited variables from the parent pipeline are treated as trigger variables, which gives them highest priority, and are therefore not overridable.

I now see that this behaviour has been discussed in several issues:

• #357275 (comment 1041853426)
• &6565
• #214340 (closed)

This behaviour is however not well documented in our docs. This MR adds the missing documentation.

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