Bluetooth: Add documentation to exported functions in lib

Most functions in `net/bluetooth/lib.c` lack propper
documentation.

This patch adds documentation to all exported functions
in `net/bluetooth/lib.c`.
Unnecessary or redundant comments are also removed to
ensure the file looks clean.

Signed-off-by: Yuran Pereira <yuran.pereira@hotmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>