[IMP] Sales: Corrected/improved outdated Down Payments page

[fw-bot]

What this PR does and why it's needed

Corrects and updates the Sales Down payments page, which contained outdated and confusing
content, based on user feedback. Fixes inaccurate/redundant explanations of down payment behavior,
cleans up UI-reference markup for consistency, and adds an LLM-readable meta description to the page.

Changes

Content corrections

• Clarified the explanation of 100% down payments, replacing a duplicated/confusing pair of
  paragraphs with a single accurate statement that another invoice is still required to complete
  the sales order.
• Removed an outdated claim that a down payment account can be set on a product category and
  prioritized for future down payments, which no longer reflects current behavior.
• Minor wording corrections throughout (e.g. "fixed dollar amounts and partial percentage down
  payments" instead of "fixed amounts and partial down payments").

Markup consistency

• Standardized references to pop-up windows, tabs, and form titles (e.g. Create invoice(s),
  Invoice Lines, Order Lines, Pay, Search: Account) to italics instead of :guilabel:,
  reserving :guilabel: for actual clickable buttons and fields.

Discoverability

• Added a .. meta:: description to the page summarizing down payment invoicing (fixed vs.
  percentage), examples, and income account modification, for LLM readability.

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This 19.0 PR can be FWP up to master.

Forward-Port-Of: #19888

[robodoo]

[fw-bot]

This PR targets saas-19.1 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port