document `[noinit]` attribute

[Delta456]

[noinit] is an attribute which doesn't allow public structs to be defined implicitly and struct members have to be initialised. In their own module, they do not have such restrictions so errors won't be given.

Currently, this behaviour is not at all documented anywhere whether it be in error messages or docs.

[fla-t]

I am interested to work on this, but I need more information.