[Format] Clarify CRC computation

[asfimport]

The format spec on CRC checksumming felt ambiguous when trying to implement it in Parquet C++, so we should make the wording clearer.

(see discussion on #126 (comment) and below)

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

Related issues:

• Clarify CRC checksum in page header (is caused by)

_{Note: This issue was originally created as PARQUET-2218. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Fixed by PR #188