Comment improvements for ForeignPath.

It's not necessarily just scanning a base relation any more.

Amit Langote and Etsuro Fujita

src/backend/optimizer/README

@@ -355,7 +355,7 @@ RelOptInfo      - a relation or joined relations
   BitmapHeapPath - top of a bitmapped index scan
   TidPath       - scan by CTID
   SubqueryScanPath - scan a subquery-in-FROM
-  ForeignPath   - scan a foreign table
+  ForeignPath   - scan a foreign table, foreign join or foreign upper-relation
   CustomPath    - for custom scan providers
   AppendPath    - append multiple subpaths together
   MergeAppendPath - merge multiple subpaths, preserving their common sort order

src/include/nodes/relation.h

@@ -1030,7 +1030,8 @@ typedef struct SubqueryScanPath
 } SubqueryScanPath;
 
 /*
- * ForeignPath represents a potential scan of a foreign table
+ * ForeignPath represents a potential scan of a foreign table, foreign join
+ * or foreign upper-relation.
  *
  * fdw_private stores FDW private data about the scan.  While fdw_private is
  * not actually touched by the core code during normal operations, it's