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`](https://github.com/joyent/node/blob/master/doc/api/assert.markdown)
- [`/doc/api/util.markdown`](https://github.com/joyent/node/blob/master/doc/api/util.markdown)

This word was injected in 192192a0.