cmd/golangorg: re-add documentation for flat presentation mode

The documentation of the "flat" presentation mode was removed
inadvertently as part of post-CLI-removal documentation cleanup
in CL 143077. The FlatDir flag still exists and is supported.
This change re-adds it to cmd/golangorg documentation.

Also fix a typo in the "src" presentation mode description.

(This is a re-apply of CL 162984 into x/website.)

Updates golang/go#25443
Updates golang/go#29206

Change-Id: I84145bf38f008678a8d8f3d3346f646aa7811886
Reviewed-on: https://go-review.googlesource.com/c/website/+/197721
Reviewed-by: Andrew Bonventre <andybons@golang.org>
diff --git a/cmd/golangorg/doc.go b/cmd/golangorg/doc.go
index 742507e..469fb82 100644
--- a/cmd/golangorg/doc.go
+++ b/cmd/golangorg/doc.go
@@ -82,7 +82,8 @@
 
 	all	show documentation for all declarations, not just the exported ones
 	methods	show all embedded methods, not just those of unexported anonymous fields
-	src	show the original source code rather then the extracted documentation
+	src	show the original source code rather than the extracted documentation
+	flat	present flat (not indented) directory listings using full paths
 
 For instance, https://golang.org/pkg/math/big/?m=all shows the documentation
 for all (not just the exported) declarations of package big.