Add help for new structured DNS framework

DNS Test Day shown that the new RR specific DNS options and the
concepts behind them may not be easily understood. This patch adds
an explanation of the new DNS framework for structured options
to make it easier for the user to understand and use the new
options.

https://fedorahosted.org/freeipa/ticket/2382

ipalib/plugins/dns.py

@@ -39,6 +39,33 @@ Domain Name System (DNS)
 
 Manage DNS zone and resource records.
 
+
+USING STRUCTURED PER-TYPE OPTIONS
+
+There are many structured DNS RR types where DNS data stored in LDAP server
+is not just a scalar value, for example an IP address or a domain name, but
+a data structure which may be often complex. A good example is a LOC record
+[RFC1876] which consists of many mandatory and optional parts (degrees,
+minutes, seconds of latitude and longitude, altitude or precision).
+
+It may be difficult to manipulate such DNS records without making a mistake
+and entering an invalid value. DNS module provides an abstraction over these
+raw records and allows to manipulate each RR type with specific options. For
+each supported RR type, DNS module provides a standard option to manipulate
+a raw records with format --<rrtype>-rec, e.g. --mx-rec, and special options
+for every part of the RR structure with format --<rrtype>-<partname>, e.g.
+--mx-preference and --mx-exchanger.
+
+When adding a record, either RR specific options or standard option for a raw
+value can be used, they just should not be combined in one add operation. When
+modifying an existing entry, new RR specific options can be used to change
+one part of a DNS record, where the standard option for raw value is used
+to specify the modified value. The following example demonstrates
+a modification of MX record preference form 0 to 1 in a record without
+modifying the exchanger:
+ipa dnsrecord-mod --mx-rec="0 mx.example.com." --mx-preference=1
+
+
 EXAMPLES:
 
  Add new zone:
@@ -960,23 +987,23 @@ class LOCRecord(DNSRecord):
             values=(u'N', u'S',),
         ),
         Int('lon_deg',
-            label=_('Degrees Longtitude'),
+            label=_('Degrees Longitude'),
             minvalue=0,
             maxvalue=180,
         ),
         Int('lon_min?',
-            label=_('Minutes Longtitude'),
+            label=_('Minutes Longitude'),
             minvalue=0,
             maxvalue=59,
         ),
         Decimal('lon_sec?',
-            label=_('Seconds Longtitude'),
+            label=_('Seconds Longitude'),
             minvalue='0.0',
             maxvalue='59.999',
             precision=3,
         ),
         StrEnum('lon_dir',
-            label=_('Direction Longtitude'),
+            label=_('Direction Longitude'),
             values=(u'E', u'W',),
         ),
         Decimal('altitude',