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.

doc/api/documentation.markdown

@@ -67,7 +67,7 @@ unlikely to ever have to change.
 ```
 
 ```
-Stability: 5 - API Locked
+Stability: 5 - Locked
 Unless serious bugs are found, this code will not ever
 change.  Please do not suggest changes in this area; they will be refused.
 ```