encoding/gob: warn about decoding data from untrusted sources

And some double space after period cleanup while I'm here.
I guess my previous regexps missed these. My next cleaner should
probably use go/ast instead of perl.

Updates #20221

Change-Id: Idb051e7ac3a7fb1fb86e015f709e32139d065d92
Reviewed-on: https://go-review.googlesource.com/47094Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/encoding/gob/decoder.go

@@ -19,6 +19,10 @@ const tooBig = 1 << 30
 
 // A Decoder manages the receipt of type and data information read from the
 // remote side of a connection.
+//
+// The Decoder does only basic sanity checking on decoded input sizes,
+// and its limits are not configurable. Take caution when decoding gob data
+// from untrusted sources.
 type Decoder struct {
 	mutex        sync.Mutex                              // each item must be received atomically
 	r            io.Reader                               // source of the data