Changes:
* Condenses and relocates the `docvalue_fields` example to the 'Run a search'
page.
* Adds docs for the `docvalue_fields` request body parameter.
* Updates several related xrefs.
Co-authored-by: debadair <debadair@elastic.co>
Moves the source filtering example snippets form the "Request body
search" API docs page to the "Return fields in a search" section of the
"Run a search" page.
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.
