bigtable: document behavior of passing multiple RowFilters

Fixes #263

Change-Id: I25fc2c34f528d177e3c7873b4cf836e6f2a2105c
Reviewed-on: https://code-review.googlesource.com/15771
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

bigtable/bigtable.go

@@ -396,6 +396,9 @@ type ReadOption interface {
 }
 
 // RowFilter returns a ReadOption that applies f to the contents of read rows.
+//
+// If multiple RowFilters are provided, only the last is used. To combine filters,
+// use ChainFilters or InterleaveFilters instead.
 func RowFilter(f Filter) ReadOption { return rowFilter{f} }
 
 type rowFilter struct{ f Filter }