app, libgimp, pdb: fix markdown-like syntax.

We need 2 lines in the PDB for it to be an empty line in the C file,
which in turn will be interpreted as markdown-like lists for the
gi-docgen generated HTML docs.

app/pdb/layer-cmds.c

@@ -1605,6 +1605,7 @@ register_layer_procs (GimpPDB *pdb)
                                   "Create a layer mask for the specified layer.",
                                   "This procedure creates a layer mask for the specified layer.\n"
                                   "Layer masks serve as an additional alpha channel for a layer. Different types of masks are allowed for initialisation:\n"
+                                  "\n"
                                   "- white mask (leaves the layer fully visible);\n"
                                   "- black mask (gives the layer complete transparency);\n"
                                   "- the layer's alpha channel (either a copy, or a transfer, which leaves the layer fully visible, but which may be more useful than a white mask);\n"

libgimp/gimplayer_pdb.c

@@ -492,6 +492,7 @@ gimp_layer_set_offsets (GimpLayer *layer,
  * This procedure creates a layer mask for the specified layer.
  * Layer masks serve as an additional alpha channel for a layer.
  * Different types of masks are allowed for initialisation:
+ *
  * - white mask (leaves the layer fully visible);
  * - black mask (gives the layer complete transparency);
  * - the layer's alpha channel (either a copy, or a transfer, which

pdb/groups/layer.pdb

@@ -293,6 +293,7 @@ This procedure creates a layer mask for the specified layer.
 Layer masks serve as an additional alpha channel for a layer. Different
 types of masks are allowed for initialisation:
 
+
 - white mask (leaves the layer fully visible);
 
 - black mask (gives the layer complete transparency);