[17.0+][IMP] invoicing: document the operation types accepted by each transport document

[joaoc-exo]

Text-only update to the PT+ transport documents documentation (no new screenshots), Odoo 18.0 branch. Supersedes the PR opened earlier today from a branch with the old naming, which GitHub closed on rename.

• Documentos Fiscais: after the series list, a short table of which operation types each transport document accepts (GR/GT/GC: internal transfers and deliveries; GA: everything except deliveries; GD: deliveries only, as the document for returns to suppliers), and a sentence in the asset transport note section about issuing it on the reception that brings goods back from a subcontractor.
• Stock: a note in the operation types section explaining that a reception carries no transport document by default and can be opted in by selecting a series on it; and an important block in the return notes how-to clarifying that the GD is for returns to suppliers (delivery operations), while customer returns carry the customer's own note.

Matches the behaviour shipped in ptplus_stock 5.1.3 and the follow-up fixes of June 2026 (task #4919).

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https://claude.ai/code/session_01VcmUPoQFknaUDute4vKsqj