update docs for heterognn support (#425)

Author: askorupka

docs/src/api/conv.md

@@ -25,15 +25,15 @@ The table below lists all graph convolutional layers implemented in the *GraphNe
 | [`GATConv`](@ref)           |          |           |     ✓       |       ✓      |              ✓             |
 | [`GATv2Conv`](@ref)         |          |           |     ✓       |       ✓      |             ✓              |
 | [`GatedGraphConv`](@ref)    |     ✓    |           |             |              |            ✓               |
-| [`GCNConv`](@ref)           |     ✓    |     ✓     |             |              |                            |
+| [`GCNConv`](@ref)           |     ✓    |     ✓     |             |       ✓      |                            |
 | [`GINConv`](@ref)           |     ✓    |           |             |       ✓      |               ✓           |
 | [`GMMConv`](@ref)           |          |           |     ✓       |              |                            |
 | [`GraphConv`](@ref)         |     ✓    |           |             |       ✓      |              ✓             |   
 | [`MEGNetConv`](@ref)        |          |           |     ✓       |              |                            |              
 | [`NNConv`](@ref)            |          |           |     ✓       |              |                            |
 | [`ResGatedGraphConv`](@ref) |          |           |             |       ✓      |               ✓             |
 | [`SAGEConv`](@ref)          |     ✓    |           |             |       ✓      |             ✓               |
-| [`SGConv`](@ref)            |     ✓    |           |             |       ✓      |             ✓             |
+| [`SGConv`](@ref)            |     ✓    |           |             |              |             ✓             |
 | [`TransformerConv`](@ref)   |          |           |     ✓       |              |                           |