Merge #10090: Update bitcoin.conf with example for pruning

Summary:
c0651cc Update bitcoin.conf with example for pruning (Kyle Honeycutt)

Tree-SHA512: f27180ac5d5a4bd32c7a63de156ca14eb8068509e64d386ca84ee16d0dacfa8e1bab9a8e7b88175fae12c8d823f71f8705d413f224a15d5aa7cf059f416fa023

Backport of Core PR10090

https://github.com/bitcoin/bitcoin/pull/10090/files

Test Plan:

echo "prune=1000" >> bitcoin.conf
bitcoind
bitcoin-cli getblockchaininfo
# verify "automatic_pruning": true,
# verify "prune_target_size": 1048576000,

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3281