Harmonise language for which header fields are permitted in which records

[JustAnotherArchivist]

Currently, a few different phrases are used:

• All records shall have a WARC-Record-ID field.
• The WARC-IP-Address field may be used on ‘response’, ‘resource’, ‘request’, ‘metadata’, and ‘revisit’ records, but shall not be used on ‘warcinfo’, ‘conversion’ or ‘continuation’ records.
• The WARC-Profile field is mandatory on ‘revisit’ type records and undefined for other record types.
• (From WARC-Segment-Number:) In the first segment of any record that is completed in one or more later ‘continuation’ WARC records, this parameter is mandatory. [...]

One additional minor quirk: sometimes, fields may be used on and sometimes in records.

The WARC-Profile one are arguably the most problematic one: 'undefined' can mean either 'shall not be used' (which is surely the intent?) or 'its meaning if present is undefined'.

I think it would be beneficial to use the same language everywhere.