docs: explain prop update breaking change

[Rich-Harris]

extracting this out of #11495 since it's unrelated. fixes #11448

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[gterras]

I believe this fixes #11448 ?

[Rich-Harris]

indeed it does — thanks!

[dummdidumm]

I honestly have no idea how this ended up in the other PR. Probably a quick draft I wrote and accidentally committed.
Still want to quickly see whether it's feasible to fix this in legacy mode.

[Rich-Harris]

Definitely feasible, we'd just need to create deriveds for member expressions as well as expressions involving function calls

[Rich-Harris]

Opened #11571 which implements the fix, meaning legacy mode components continue to work the same way. We still need this PR though, since the behaviour changes in runes mode (unless we wanted to wrap props in deriveds more aggressively there too, but I don't think we do)

[dummdidumm]

Closing since we merged the fix