[DOC] Fix the rdoc for File::Stat#size? [ci skip]

file.c

@@ -6063,11 +6063,13 @@ rb_stat_z(VALUE obj)
 
 /*
  *  call-seq:
- *     state.size    -> integer
+ *     stat.size?    -> Integer or nil
  *
- *  Returns the size of <i>stat</i> in bytes.
+ *  Returns +nil+ if <i>stat</i> is a zero-length file, the size of
+ *  the file otherwise.
  *
- *     File.stat("testfile").size   #=> 66
+ *     File.stat("testfile").size?   #=> 66
+ *     File.stat("/dev/null").size?  #=> nil
  *
  */