... also function wiced_platform_register_button_callback.
Does exist some criteria, according to which some items from SDK are documented, others - NOT.
You can find the definition of these in wiced_platform.c which is located in baselib folder inside wiced_btsdk.
I would suggest you to utilize search functionality (do ctrl+H). Take C/C++ search tab in it, set the scope and click on search.
After the search just right click on the structure/function name and open the declaration.
See the attached screenshot.
Capture.JPG 260.7 K