Argon2: Fix Params docs

[sorairolake]

• The minimum value of m_cost is 8 ( 8*MIN_OUTPUT_LEN), not 1.
• The maximum value of p_cost is (2^24)-1, not 255.

These are defined in RFC 9106, and MIN_M_COST and MAX_P_COST are also like that.

See also: #451, #452

[tarcieri]

I verified these match the RFC.

[sorairolake]

Params::new verifies that m_cost is greater than or equal to p_cost*8.

password-hashes/argon2/src/params.rs

Lines 119 to 121 in 144b2e4

	if m_cost < p_cost * 8 {
	return Err(Error::MemoryTooLittle);
	}

So, it might be better to explain that the minimum value of m_cost is p_cost*8 instead of 8.