Editing User Guide grammar, clarity

This issue is a placeholder for reviewing and editing the User Guide to improve readability by checking for grammatical mistakes and editing for clarity. (I think the list below is in the order that the doc is assembled, with of course the dhcpv4 and dhcpv6 server parts broken down into 2000 line chunks)

• kea-guide.xml intro.xml quickstart.xml install.xml admin.xml config.xml
• keactrl.xml
• dhcpv4-serv lines 1 - 2000
• dhcpv4-serv lines 2000 - 4000
• dhcpv4-serv lines 4000 - end
• dhcpv6-serv lines 1 - 2000
• dhcpv6-serv lines 2000 - 4000
• dhcpv6-serv lines 4000 - end
• lease-expiration.xml logging.xml
• ddns.xml hooks.xml hooks-class-cmds.xml hooks-ha.xml, hooks-host-cache.xml
• hooks-lease-cmds.xml hooks-radius.xml hooks-stat-cmds.xml libdhcp.xml
• lfc.xml stats.xml ctrl-channel.xml classify.xml shell.xml agent.xml
• netconf.xml api.xml congestion-handling.xml