improve and correct documentation on indices analyze api

I encountered this error
`org.elasticsearch.common.netty.handler.codec.frame.TooLongFrameException: An HTTP line is larger than 4096 bytes.`
analyzing a long piece of text.
Using the :body parameter this works fine. I thought it worth adding a note to the docs.

The change of documented type of the :body parameter is actually already covered by a test in analyze_test.rb
`subject.indices.analyze :body => 'foo'`