Documentation for Elgg 1.9

Updated function names to be compatible with Elgg 1.9, added information about migration and link to source code.
This syntax was needed to be able to link to the Elgg code reference. Now that the function is not found from the reference anymore, these functions should be written as regular inline code blocks.

So could you make the following changes to all the three functions:
```...using `expose\_function()`. To enable...```
```...using ``expose_function()``. To enable...```