docs: Clean up LDAP docs a bit more.
docs: Clean up LDAP docs a bit more.

In particular, this improves:
* The explanation of how data is mapped into Zulip
* The explanation of what is printed out by `manage.py query_ldap`
* Makes sure users create their first account with EmailAuthBackend.

jackrzhang/zulip,punchagan/zulip,rishig/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,eeshangarg/zulip,timabbott/zulip,kou/zulip,shubhamdhama/zulip,dhcrzf/zulip,showell/zulip,zulip/zulip,brainwane/zulip,dhcrzf/zulip,shubhamdhama/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,zulip/zulip,tommyip/zulip,punchagan/zulip,punchagan/zulip,jackrzhang/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,rishig/zulip,rht/zulip,brainwane/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,jackrzhang/zulip,shubhamdhama/zulip,jackrzhang/zulip,timabbott/zulip,tommyip/zulip,zulip/zulip,jackrzhang/zulip,kou/zulip,zulip/zulip,rishig/zulip,zulip/zulip,rishig/zulip,brainwane/zulip,timabbott/zulip,showell/zulip,eeshangarg/zulip,jackrzhang/zulip,dhcrzf/zulip,shubhamdhama/zulip,punchagan/zulip,zulip/zulip,rht/zulip,kou/zulip,andersk/zulip,dhcrzf/zulip,rht/zulip,shubhamdhama/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,dhcrzf/zulip,andersk/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,showell/zulip,jackrzhang/zulip,tommyip/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,kou/zulip,hackerkid/zulip,timabbott/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,dhcrzf/zulip,rishig/zulip,dhcrzf/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,tommyip/zulip,synicalsyntax/zulip,showell/zulip,rishig/zulip,timabbott/zulip,rht/zulip,synicalsyntax/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,rishig/zulip