Add is_coalesced to GNNGraphs docs (#624)

Should this Symbol be exported?

Author: simsurace

GNNGraphs/docs/src/api/gnngraph.md

@@ -37,6 +37,7 @@ Private = false
 
 ```@docs; canonical = true
 Graphs.neighbors(::GNNGraph, ::Integer)
+GNNGraphs.is_coalesced(::GNNGraph)
 ```
 
 ## Transform