Documentation for CRC util

[michael-etzkorn]

Type of issue: other enhancement

Impact: enhanced user experience

Development Phase: proposal

Other information

What is the current behavior?
User is directed to CRC zoo where implicit 1 notation is used but CRC object uses explicit 1 notation which can cause unexpected behavior if require is not hit.
What is the expected behavior?

Document that this is explicit 1 notation and that crc zoo uses implicit 1 notation.

What is the use case for changing the behavior?
This was only discoverable while reading the code and noticing the require. This'll prevent user misuse and headache.

https://github.com/chipsalliance/rocket-chip/blob/master/src/main/scala/util/CRC.scala#L14