Pass pruneblockchain RPC Results and Examples to RPCHelpMan

Summary:
Accidentally skipped over this one when backporting the changes to the
other RPCs in blockchain.cpp.

Partial backport of Core PR14987

Test Plan:
On master:

ninja
./bitcoind
./bitcoin-cli help > RPCs
./getrpchelps.sh RPCs master

Then on the patched branch:

ninja
./bitcoind
./getrpchelps.sh RPCs patched
./checkrpc.sh RPCs master patched pruneblockchain.diff

Verify pruneblockchain.diff does not exist.

Scripts:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5921