As far as I know, any metaclass in UML can be extended with stereotypes.
The metaclass is LinkEndData, not LinkEnd. LinkEndDatas are used in actions that apply to links (i.e. instances of associations), namely ReadLinkAction, CreateLinkAction and DestroyLinkAction.
Btw, I strongly advise you should have the org.eclipse.uml2.uml plugin in your workspace or your target, it makes it very easy to find out more about any metaclass - just hit Ctrl-T and type the name to open the corresponding UML2 class.
My fault, I meant Ctrl-Shift-T or the Open Type dialog (Navigate > Open Type... in the Java perspective, for instance), which opens any Java type in your workspace.
The UML 2.x specification is available as a PDF file from the OMG site. But if you are in Eclipse, it is much much easier to access it by opening the corresponding Java class from the UML2 plug-in and reading the Javadoc (I use the Javadoc view for better readability).
Alternatively, you can read the javadoc from the UML2 plug-in documentation, but is not nearly as convenient: