[Docs] Improve password_hash() documentation, specifically about Argon2

[galvao]

Affected page

password_hash
predefined constants

Current issue

Lack of clarity:

• memory_cost option reads: "Maximum memory (in kibibytes) that may [...]", denoting a clear typo: is it kilobytes? That was my first guess, but then again, the predefined constant for memory cost states that it is provided in bytes, so which one is it?
• time_cost: There's no information about which unit of time is used (probably seconds, but this is a guess). The information is also missing from the predefined constant for time cost.

Suggested improvement

Provide the correct information about which unit is used to set the costs, both for memory and time.
It would probably be a good idea to double check the remaining documentation for password_* functions.