Improve documentation for Module.get_attribute

[khia]

The following special case probably deserve some documenting
https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/module.ex#L820

[ericmj]

The warnings are only used internally afaik. Did you have something specific in mind?

[khia]

This example is outdated:
https://github.com/elixir-lang/elixir/blob/master/lib/elixir/lib/module.ex#L771
It crashes with something like:

** (FunctionClauseError) no function clause matching in Module.get_attribute/3

That is the reason for confusion. However nil works as expected.

[ericmj]

Thanks for the report. Fixed on master.

[josevalim]

This functionality should be private. I have pushed a commit that makes it so!