sources.list(5): Explicitly say what the precedence order is

Simon McVittierequested to merge
smcv/apt-branches:sources-precedence into main

The order of apt sources can matter, but this man page did not previously define whether /etc/apt/sources.list was parsed before or after the fragment files in sources.list.d. Giving a concrete example also provides a hint that numeric prefixes are a useful way to force a particular sort order, and clarifies that the .list or .sources extension is less important than lexicographic order.

Similar to !416 (merged) but for sources.list(5).

Edited by Simon McVittie

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