Improve guidance on removing default mappings.

docs/reference/migration/migrate_7_0/mappings.asciidoc

@@ -31,6 +31,20 @@ of `_id`.
 The `_default_` mapping has been deprecated in 6.0 and is now no longer allowed
 in 7.0. Trying to configure a `_default_` mapping on 7.x indices will result in
 an error.
+
+If an index template contains a `_default_` mapping, it will fail to create new
+indices. To resolve this issue, the `_default_` mapping should be removed from
+the template. Note that in 7.x, the <<indices-get-template, get template API>>
+does not show the `_default_` mapping by default, even when it is defined in
+the mapping. To see all mappings in the template, the `include_type_name`
+parameter must be supplied:
+
+```
+GET /_template/my_template?include_type_name
+```
+
+For more details on the `include_type_name` parameter and other types-related
+API changes, please see <<removal-of-types>>.
 //end::notable-breaking-changes[]
 
 [float]

server/src/main/java/org/elasticsearch/index/mapper/MapperService.java

@@ -130,6 +130,9 @@ public enum MergeReason {
     private static final ObjectHashSet<String> META_FIELDS = ObjectHashSet.from(SORTED_META_FIELDS);
 
     private static final DeprecationLogger deprecationLogger = new DeprecationLogger(LogManager.getLogger(MapperService.class));
+    static final String DEFAULT_MAPPING_ERROR_MESSAGE = "[_default_] mappings are not allowed on new indices and should no " +
+        "longer be used. See [https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-7.0.html" +
+        "#default-mapping-not-allowed] for more information.";
 
     private final IndexAnalyzers indexAnalyzers;
 
@@ -449,11 +452,9 @@ private synchronized Map<String, DocumentMapper> internalMerge(@Nullable Documen
 
         if (defaultMapper != null) {
             if (indexSettings.getIndexVersionCreated().onOrAfter(Version.V_7_0_0)) {
-                throw new IllegalArgumentException("The [default] mapping cannot be updated on index [" + index().getName() +
-                        "]: defaults mappings are not useful anymore now that indices can have at most one type.");
+                throw new IllegalArgumentException(DEFAULT_MAPPING_ERROR_MESSAGE);
             } else if (reason == MergeReason.MAPPING_UPDATE) { // only log in case of explicit mapping updates
-                deprecationLogger.deprecated("[_default_] mapping is deprecated since it is not useful anymore now that indexes " +
-                        "cannot have more than one type");
+                deprecationLogger.deprecated(DEFAULT_MAPPING_ERROR_MESSAGE);
             }
             assert defaultMapper.type().equals(DEFAULT_MAPPING);
             results.put(DEFAULT_MAPPING, defaultMapper);

server/src/test/java/org/elasticsearch/index/mapper/LegacyMapperServiceTests.java

@@ -86,7 +86,7 @@ public void testDefaultMappingIsDeprecatedOn6() throws IOException {
         }
         final MapperService mapperService = createIndex("test", settings).mapperService();
         mapperService.merge("_default_", new CompressedXContent(mapping), MapperService.MergeReason.MAPPING_UPDATE);
-        assertWarnings("[_default_] mapping is deprecated since it is not useful anymore now that indexes cannot have more than one type");
+        assertWarnings(MapperService.DEFAULT_MAPPING_ERROR_MESSAGE);
     }
 
 }

server/src/test/java/org/elasticsearch/index/mapper/MapperServiceTests.java

@@ -313,8 +313,7 @@ public void testDefaultMappingIsRejectedOn7() throws IOException {
         MapperService mapperService = createIndex("test").mapperService();
         IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
                 () -> mapperService.merge("_default_", new CompressedXContent(mapping), MergeReason.MAPPING_UPDATE));
-        assertEquals("The [default] mapping cannot be updated on index [test]: defaults mappings are not useful anymore now"
-            + " that indices can have at most one type.", e.getMessage());
+        assertEquals(MapperService.DEFAULT_MAPPING_ERROR_MESSAGE, e.getMessage());
     }
 
     public void testFieldNameLengthLimit() throws Throwable {