crypto: add missing docs

[virus-rpi]

This pull request introduces several documentation improvements across multiple files in the vlib/crypto directory.

• vlib/crypto/cipher/cfb.v: Added a comment to the xor_key_stream function explaining its purpose.
• vlib/crypto/cipher/ctr.v: Added a comment to the xor_key_stream function explaining its purpose.
• vlib/crypto/cipher/ofb.v: Added a comment to the xor_key_stream function explaining its purpose.
• vlib/crypto/des/des.v: Added comments to the encrypt and decrypt functions in both DesCipher and TripleDesCipher classes explaining their purposes. [1] [2] [3] [4]
• vlib/crypto/rand/rand.v: Added a comment to the msg function in the ReadError struct explaining its purpose.

[huly-for-github]

Connected to Huly®: V_0.6-22268

[spytheman]

Are you sure that each byte is xor-ed?
To me it is not obvious 🤔 .

@blackshirt, @kimshrier what do you think?

[spytheman]

I just saw the comments in the interface:

// A Stream represents a stream cipher.
pub interface Stream {
mut:
    // xor_key_stream XORs each byte in the given slice with a byte from the
    // cipher's key stream. Dst and src must overlap entirely or not at all.
    //
    // If len(dst) < len(src), xor_key_stream should panic. It is acceptable
    // to pass a dst bigger than src, and in that case, xor_key_stream will
    // only update dst[:len(src)] and will not touch the rest of dst.
    //
    // Multiple calls to xor_key_stream behave as if the concatenation of
    // the src buffers was passed in a single run. That is, Stream
    // maintains state and does not reset at each xor_key_stream call.
    xor_key_stream(mut dst []u8, src []u8)
}

[spytheman]

Given that method implements that Stream interface, I think it is safe to assume that the same comments should apply.

[spytheman]

I'll merge it as it is for now.

[spytheman]

Excellent work.