Fix small docs inconsistencies
The patch with some small-ish fixes:
- Enumerator refers to
Kernel, while in fact they are documented under
Kernel#warnhas outdated call sequence;
ClosedQueueErroris not mentioned in errors hierarchy;
thenis referred to as
yield_self, obscuring the usage.
Not fixed yet, though should be:
ClosedQueueErrordoesn't renders its parent right (probably RDoc bug also, will investigate).
For the next, step, I plan to provide a PRs with core and stdlib docs that are just missing, outdated or confusing.