doc: clarify pruning impact on wallet sync #35076

This updates prune-related help text to document the existing caveat that wallets should remain loaded, and relevant indexes should remain active, while pruning is enabled. This follows the behavior discussed in #30671 without changing startup, pruning, wallet, or index behavior.

Refs #30671.

Testing:

• cmake -S . -B build -DBUILD_GUI=OFF -DBUILD_TESTS=OFF -DBUILD_BENCH=OFF -DBUILD_FUZZ_BINARY=OFF -DENABLE_IPC=OFF -DCMAKE_PREFIX_PATH=/opt/homebrew
• cmake --build build --target bitcoind -j$(sysctl -n hw.ncpu)
• ./build/bin/bitcoind -help 2>&1 | rg -A10 -- '-prune=<n>'
• xmllint --noout src/qt/forms/optionsdialog.ui
• git diff --check

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34435 (refactor: use _MiB/_GiB consistently for byte conversions by l0rinc)

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