binder: Fix missing kernel-doc entries in binder.c

Fix several kernel-doc warnings in `drivers/android/binder.c` caused by
undocumented struct members and function parameters.

In particular, add missing documentation for the `@thread` parameter in
binder_free_buf_locked().

Signed-off-by: Kriish Sharma <kriish.sharma2006@gmail.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>