Add docs about running TF with builds from other PR

[lbarcziova]

Related to packit/packit-service#1658

[softwarefactory-project-zuul[bot]]

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[softwarefactory-project-zuul[bot]]

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[lbarcziova]

More comments on 11 lines doc change than on 1.7k line code change :see_no_evil:

:D I was definitely more passionate about writing the code than the docs, so that may be the reason

My main point here is to explain the feature in more detail so that packaging beginners can understand the use case.

Sure, you are right, thanks for suggestions!

[softwarefactory-project-zuul[bot]]

Build succeeded.

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[softwarefactory-project-zuul[bot]]

Build succeeded (gate pipeline).

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