Add some documentation for the advanced search
Add some documentation for the advanced search

Includes some discussion of how its implemented, some examples of how
to use it, and a link to more details of the search language syntax.

T2DREAM/t2dream-portal,ENCODE-DCC/snovault,ENCODE-DCC/encoded,T2DREAM/t2dream-portal,T2DREAM/t2dream-portal,ENCODE-DCC/encoded,T2DREAM/t2dream-portal,ENCODE-DCC/snovault,ENCODE-DCC/encoded,ENCODE-DCC/snovault,ENCODE-DCC/snovault,ENCODE-DCC/encoded,ENCODE-DCC/snovault