doc: add missing top-level description to pruneblockchain RPC #32330

Previously, the pruneblockchain RPC help output included only the method signature and arguments, with no top-level description explaining its purpose or constraints.

This adds a concise description noting that the method deletes block and undo data, requires -prune to be enabled at startup, and is irreversible.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32330.

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