x/playground: Maybe warn user if import of "time" or "net" is seen

[gopherbot]

The playground just keeps confusing people. Here's an idea to make it's intentional
limitations more discoverable:

- add a content slot just before output, that is marked up with e.g.  background color
that differentiates it from output, that can contain messages; make it invisible if empty

- if the code imported "time", add message like "Playground is frozen in
time to 2009."

- if the code imported "net", add message like "Playground has fake
networking that can only communicate within the application." 

- if either of the two messages triggered, add a link "See About for more."

[minux]

Comment 1:

I don't think it's a good way to convey the info to the user.
what we can probably do is expand the about by default on the right side
when the user first vists that site, and if the user has closed it, it will
never be shown again by default.

Labels changed: added release-none, repo-playground.

Owner changed to @adg.

[adg]

Comment 2:

Some people do get confused, but how many and how often?
We don't observe the many people who read the About page and are not confused.
Not convinced we actually need to do something here.

[gopherbot]

Comment 3:

Of course I don't have numbers, and the search at https://botbot.me/freenode/go-nuts/
does not seem good enough to do multiple keyword matching.
Playground time is definitely one of the recurring topics on #go-nuts.
Do you *really* think people read the About page? You have so much faith in humanity..

[adg]

Comment 4:

If someone want to design some UI for this that is unobtrusive and can be dismissed
(forever) once per browser, I'd be willing to review the change.

Status changed to Accepted.