pruning docs need update

[nhooyr]

https://github.com/golang/dep/blob/70b4da73f4dc8160938512f49a9773b6224b8b1b/docs/Gopkg.toml.md#prune

Pruning is disabled by default. It can be enabled by setting them to true at the root level.

Pruning is now enabled by default.

[ibrasho]

Actually, this refers to the default value for pruning options.

If you don't add [prune] and enable some options (go-tests, unused-packages ... etc) they are all false by default.

[nhooyr]

That’s really misleading IMO. The docs should clarify that pruning is disabled by default but dep init will create a new Gopkg.toml with pruning enabled on unused packages and go-tests

[jmank88]

Related to #1650. We can keep working on clarifying the phrasing there.