cmd/godoc: 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/godoc documentation.

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

Updates golang/go#25443

Change-Id: If4705d70bc5eafee92cdfd52409aca4606a0efd7
Reviewed-on: https://go-review.googlesource.com/c/162984
Reviewed-by: Agniva De Sarker <agniva.quicksilver@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/godoc/doc.go

@@ -89,7 +89,8 @@ The presentation mode of web pages served by godoc can be controlled with the
 
 	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.