FIXED: Docs should specify the default behavior when `stage` key is absent

Here’s the current version of the stage key documentation:

It doesn’t mention that when the stage key is absent snapcraft will by default stage all files in the install directory.

Also it should mention that when one wants to stage no files for this part one should use the following syntax to exclude all files:

  - -*

Reference: Error "while loading shared libraries" with BloodHound snap

I make a new topic to document it: The behavior of the stage and prime keywords