[Doc] Update "Java API Administration >> Indices Administration" page

[sfalquier]

Elasticsearch version : 6.1.1

Description :
Since PutMappingRequestBuilder.setSource(String source) method has been removed (deprecated since 5.3 in #22691) in favour of #setSource(String, XContentType), Indices Administration page in Java API documentation is obsolete :
https://www.elastic.co/guide/en/elasticsearch/client/java-api/current/java-admin-indices.html

All occurences of setSource(String source) or addMapping(String type, String source) method should be replaced with correct versions taking XContentType as parameter.

[cbuescher]

The PR adressing this needed to be reverted temporarily because it broke the docs build, reopening here as well.