Minor follow-up improvements to PR #19227 for aro translate-c

[f-cozzocrea]

Adds a few minor improvements that didn't make it into #19227. Namely:

1. Don't calculate head_field_alignment if it isn't used (ie. if there aren't any alignment attributes).
2. Doc comment added for alignmentForField() explaining why it's needed. Also removed specific references to u29 since it seems that Zig's alignment range may change in the future.