open <namespace> command does not work as expected

[lecopivo]

Calling open Lean before #doc command does not open the namespace inside of ```lean ... ``` code blocks. Example:

open Lean

#doc (Manual) "Test" =>
```lean
#check MetaM  -- unknown identifier 'MetaM'
```

Calling it inside of ```lean ... ``` works as expected

#doc (Manual) "Test" =>
```lean
#check MetaM  -- Lean.Meta.MetaM (α : Type) : Type
```