Add the backward seek support comment to fdb_iterator_seek API.
Add the backward seek support comment to fdb_iterator_seek API.

Change-Id: I86ca80172adc954957d10db589dcae3bae37a55e

couchbase/forestdb,jinxuan/forestdb,joliss/forestdb,hisundar/forestdb,nvoron23/forestdb,hisundar/forestdb,joliss/forestdb,zhaog/forestdb,zhaog/forestdb,couchbase/forestdb,wurikiji/forestdb,wurikiji/forestdb,yyangpan/forestdb,nvoron23/forestdb,sushmaad/forestdb,sushmaad/forestdb,seem-sky/forestdb,jinxuan/forestdb,seem-sky/forestdb,yyangpan/forestdb