doc: modify documentation style about 'Stability: 5'

'Stability: 5' is described as 'Locked' not as 'API Locked'
in other documents.

For example:
- `/doc/api/assert.markdown`
- `/doc/api/util.markdown`

This word was injected in 192192a.
events: Don't crash on events named __proto__

This prefixes all event names internally with 'ev'.