Documentation on the generate() output-size for HMAC-based secret key newtypes is incorrect

[brycx]

The documentation states:

https://github.com/brycx/orion/blob/41f805378d3e7318e7ba186569a08c9cee0c0a65/src/typedefs.rs#L637

This is incorrect as it will actually generate equal to the size of the underlying SHA2 blocksize (128). This comment should be made generic, such that it can be valid for the upcoming different variants of SHA2:

// Initialize a secret key with random bytes.

The length of it can always be found with the .len() function.