encoding/gob: mention that Encoder and Decoder are safe for concurrent use

[mostynb]

gob.Encoder and gob.Decoder use mutexes internally to allow concurrent usage, but I couldn't see this mentioned in the package documentation anywhere. It might be useful to do so.

[gopherbot]

Change https://golang.org/cl/155742 mentions this issue: encoding/gob: mention that this is safe for concurrent usage

[mvdan]

I always forget if we need to document when named types and methods are safe for concurrent use, or when they're not. We should probably have a wiki page explaining this.

[mostynb]

I forget too, and so I default to treating things as unsafe for concurrent use until told otherwise- which seems like the most reasonable response.

If there's general agreement on if/what/how this should be documented, I would be happy to help make this more consistent across the standard library docs.