Add a reference on returning fields during a search.

This PR adds a section to the new 'run a search' reference that explains
the options for returning fields. Previously each option was only listed as a
separate request parameter and it was hard to know what was available.

Author: jtibshirani

docs/reference/search/run-a-search.asciidoc

@@ -285,3 +285,5 @@ GET /*/_search
 ====
 
 include::request/from-size.asciidoc[]
+
+include::search-fields.asciidoc[]

docs/reference/search/search-fields.asciidoc

@@ -0,0 +1,30 @@
+[discrete]
+[[search-fields]]
+=== Return fields in a search
+
+By default, each hit in the search response includes the document
+<<mapping-source-field,`_source`>>, which is the entire JSON object that was
+provided when indexing the document. Often only certain fields are required in
+the response. In this case you can use
+<<request-body-search-source-filtering,source filtering>> to restrict what
+parts of the source are returned.
+
+Returning fields using the document source has some limitations:
+
+* The `_source` field does not include <<multi-fields, multi-fields>> or
+<<alias, field aliases>>. Likewise, a field in the source will not contain
+values that were copied into it through `copy_to`.
+* Since the `_source` is stored as a single field in Lucene, the whole source
+object must be loaded and parsed, even if only a small number of fields is needed.
+
+Elasticsearch supports some alternative methods for returning fields that help
+avoid these downsides:
+
+* The <<request-body-search-docvalue-fields, docvalue fields>>
+parameter allows for loading fields from their docvalues. This can be a good
+choice when returning a fairly small number of fields that support docvalues,
+such as keywords and dates.
+* It's also possible to store an individual field's values by using the
+<<mapping-store,`store`>> mapping option. These stored values can then be
+returned in a search through the
+<<request-body-search-stored-fields, `stored_fields`>> parameter.