net80211: add some example doxygen inline documents

[adrian]

Summary

This isn't supposed to be comprehensive, I want to have examples
for the common doxygen comments:

• describing a struct
• inside a struct - single line before the item
• inside a struct - same line after an item
• inside a struct - multi-line before an item

These build fine with the doxygen build in tools/kerneldoc/subsys/ .