spec: clarify file versus package attributes with examples

[mvdan]

CUE currently supports attributes attached to just one file, such as:

@extern(embed)

package foo

a: _ @embed(file=foo.json)

Here, @extern(embed) only affects its file, and not any other files part of the same instance of package foo.

Any top-level attributes after a package clause (or without a package clause) are attached to the package (or rather its instance), so they do get unified as part of it and affect all other files which are part of the same instance. For example:

package foo

@go(some_go_code_generation_option)

Here, the attribute will affect how e.g. cue exp gengotypes generates Go code for the entire CUE package instance.

https://cuelang.org/docs/reference/spec/#attributes needs to be expanded; it currently only explains package attributes, showing one brief example. It needs to explain file-scoped attributes too, and show an example of them too.