Clarify description of blockindex
Clarify description of blockindex

see issues:

https://github.com/bitcoin-dot-org/bitcoin.org/issues/1237

https://github.com/bitcoin/bitcoin/issues/7532

Github-Pull: #7541
Rebased-From: 7eef1d0dad0c48e9faefdc4fe5cfb6182edc79ab

adpg211/bitcoin-master,bitcoinclassic/bitcoinclassic,adpg211/bitcoin-master,zander/bitcoinclassic,bitcoinclassic/bitcoinclassic,adpg211/bitcoin-master,zander/bitcoinclassic,zander/bitcoinclassic,adpg211/bitcoin-master,deadalnix/bitcoin,zander/bitcoinclassic,bitcoinclassic/bitcoinclassic,deadalnix/bitcoin,adpg211/bitcoin-master,bitcoinclassic/bitcoinclassic,bitcoinclassic/bitcoinclassic,zander/bitcoinclassic,bitcoinclassic/bitcoinclassic,deadalnix/bitcoin,deadalnix/bitcoin,deadalnix/bitcoin,deadalnix/bitcoin,adpg211/bitcoin-master,zander/bitcoinclassic