rpc: fix mempool entry vsize docs #35898

Follow-up to #32800.

This fixes the RPC help text for MempoolEntryDescription(). These docs are used by mempool-entry RPCs such as getmempoolentry, verbose getrawmempool, getmempoolancestors, and getmempooldescendants.

The vsize fields in those results describe transactions already in the mempool, so they are always returned and there is no allowed field. This removes the incorrect optional markers and the testmempoolaccept-specific “only present when allowed is true” wording.

No behavior change.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35898.

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ACK a3ebf8ab607f2529b634eb6fa990aef7def43d1d

cc @hebasto given this has (necessarily) changed some strings.

cc @hebasto given this has (necessarily) changed some strings.

None of the modified strings seem to be part of the translations.