fix yard doc warnings

````
ffi.git master $ yardoc
[warn]: @param tag has unknown parameter name: [] 
    in file `lib/ffi/types.rb' near line 161
[warn]: @param tag has unknown parameter name: cname 
    in file `lib/ffi/library.rb' near line 317
[warn]: @param tag has unknown parameter name: type 
    in file `lib/ffi/library.rb' near line 317
[warn]: @param tag has unknown parameter name: name 
    in file `lib/ffi/library.rb' near line 368
[warn]: @param tag has unknown parameter name: params 
    in file `lib/ffi/library.rb' near line 368
[warn]: @param tag has unknown parameter name: ret 
    in file `lib/ffi/library.rb' near line 368
[warn]: @param tag has unknown parameter name: [] 
    in file `lib/ffi/library.rb' near line 407
[warn]: @param tag has unknown parameter name: ptr 
    in file `lib/ffi/autopointer.rb' near line 189
[warn]: @param tag has unknown parameter name: alignment 
    in file `lib/ffi/struct_layout_builder.rb' near line 56
Files:          38
Modules:         7 (    0 undocumented)
Classes:        49 (   19 undocumented)
Constants:      37 (   29 undocumented)
Methods:       314 (   38 undocumented)
 78.87% documented
ffi.git master $ 
````