Payment API - Add description of order_reference parameter

[mattwire]

Overview

The order_reference parameter has been supported since 5.27 but is missing from the spec.

Before

Param missing

After

Param added

Technical Details

Comments

Just makes this parameter a little bit more discoverable

[civibot]

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[totten]

I checked the metadata in both CLI and API Explorer. Looks good on all counts (r-explain, r-user, r-doc, r-run, r-tech, r-code, r-maint, r-test). Merging.

Tangential observation: This doesn't reflect on the merits of this PR, but maybe it's worth saying out loud... there are several fields (order_reference, trxn_id, trxn_date, etal) which are defined in both Payment.create and Payment.get. They do match in the most important characteristics (e.g title, type), but there are also a bunch of random-seeming discrepancies in the metadata (e.g. when do size/maxlength matter? e.g. when do table_name/bao matter? should it be a little DRYer?). It might merit a closer look sometime.... (Again, it doesn't affect this PR. The PR is a functional improvement and is consistent with the code being patched.)