cmd/go: document PackageError in go list output

Fixes #14007.

Change-Id: I1f73dfccb466d8fd00efbd8c92a31ac538bf5988
Reviewed-on: https://go-review.googlesource.com/18731Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/alldocs.go

@@ -603,6 +603,14 @@ syntax of package template.  The default output is equivalent to -f
         XTestImports []string // imports from XTestGoFiles
     }
 
+The error information, if any, is
+
+    type PackageError struct {
+        ImportStack   []string // shortest path from package named on command line to this one
+        Pos           string   // position of error (if present, file:line:col)
+        Err           string   // the error itself
+    }
+
 The template function "join" calls strings.Join.
 
 The template function "context" returns the build context, defined as:

src/cmd/go/list.go

@@ -78,6 +78,14 @@ syntax of package template.  The default output is equivalent to -f
         XTestImports []string // imports from XTestGoFiles
     }
 
+The error information, if any, is
+
+    type PackageError struct {
+        ImportStack   []string // shortest path from package named on command line to this one
+        Pos           string   // position of error (if present, file:line:col)
+        Err           string   // the error itself
+    }
+
 The template function "join" calls strings.Join.
 
 The template function "context" returns the build context, defined as: