Add more implementation guidance for thin-film iridescence

Think-film iridescence can be difficult to implement, and the paper we reference can make it even more complex due to advanced color-related transformations. This task is to add more implementation guidance to the OpenPBR spec based on our own implementations of thin-film iridescence, which are summarized on Slack:

Thin-film GLSL implementation (from @portsmouth ):
https://academysoftwarefdn.slack.com/archives/C06365EAQMP/p1710080253470669

Thin-film iridescence in Eclair (from @peterkutz ):
https://academysoftwarefdn.slack.com/archives/C06365EAQMP/p1732332598108709?thread_ts=1710080253.470669&cid=C06365EAQMP

More thin-film implementation notes: Improved iridescence on metals (from @peterkutz ):
https://academysoftwarefdn.slack.com/archives/C06365EAQMP/p1733848688599819?thread_ts=1710080253.470669&cid=C06365EAQMP

Start by locating the OpenPBR specification section covering thin-film iridescence, then review the three linked Slack implementation notes from the GLSL and Eclair implementations. Add practical guidance that makes the referenced paper easier to implement, with the spec's thin-film iridescence section updated as the completion point.

Written by the indexing model from the issue text.