Add doc examples for eq_dyn etc.

[alamb]

The comparison kernels have a somewhat bewildering array of options for a new developer

https://docs.rs/arrow/7.0.0/arrow/compute/kernels/comparison/index.html

We have been making the API easier to use with so called dyn kernels (such as eq_dyn, etc) but this may not be as discoverable as would be ideal.

Thus it would be great to add some doctests to the comparison module that demonstrated how to use those kernels with an example of creating two arrays, and then calling eq_dyn on them

[alamb]

Perhaps one could follow the example of

https://docs.rs/arrow/7.0.0/arrow/array/type.StringArray.html

use arrow::array::StringArray;
let array1 = StringArray::from(vec![Some("foo"), None, Some("bar")]);
let array2 = StringArray::from(vec![Some("foo"), None, Some("baz")]);
let result = eq_dyn(&array1, &array2).unwrap();
assert_eq!(BooleanArray::from(vec![Some(true), None, Some(false)]), result))

[HaoYang670]

I can work on it! Please assign to me @alamb

[alamb]

Thanks @HaoYang670