[IMP] inventory: operations valuation article clarification

[fw-bot]

What this PR does and why it's needed

Clarifies the "How inventory operations affect valuation" article by retitling it to better reflect its scope and by explaining the distinction between the Unit Cost shown on a stock movement and the Cost field on the product form, which can otherwise be mistaken for the same value.

Changes

Inventory Valuation

• Renamed the page title from "How inventory operations affect valuation" to "Valuation of stock movements and inventory operations" for clarity.
• Clarified that Unit Cost is specific to each stock movement, and fixed its costing-method reference link.
• Added an important note explaining that the Cost field on the product form shows the average price of the product — even under the FIFO costing method — and is affected by outbound stock movements in that case; it is informational only and not used for valuation.

Additional changes during review

These changes were made in a follow-up commit (implementing Felicia's review suggestion) and later squashed into the final commit, all within content/applications/inventory_and_mrp/inventory/inventory_valuation/operations_valuation.rst:

1. :guilabel: → italics for "Cost" field (lines 187-190)
   Changed :guilabel:Cost`` to plain italic Cost throughout the callout, since it refers to the product form field conceptually rather than as clickable UI.

2. New clarification on Cost field's role in adjustments (lines 192-194)
   Replaced the old closing line ("The Cost field is not used for valuation, and is only informational") with a more accurate statement:

   "The value in the Cost field is used as the unit price for :ref:inventory/operations_valuation/inventory-adjustments, even when using the FIFO costing method."

3. Renamed virtual location (line ~319)
   Inventory Loss location -> Inventory adjustment location, matching Odoo's actual naming.

4. Reworded valuation trigger for adjustments (lines 318-322)
   Changed from "based on the current unit cost of the product" to "based on the difference between the counted quantity and the expected quantity" - a more precise description of what drives the value change.

5. New paragraph added (lines 328-330)

   "Inventory adjustments use the product's current averaged cost as the :ref:unit cost <inventory/operations_valuation/stock-movement-reported-values> for the stock movement."

Net effect: 24 insertions, 8 deletions. The core addition clarifies that inventory adjustments always use the product's averaged cost as the unit price - even under FIFO - correcting the earlier (less precise) statement that the Cost field was purely informational.

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This 19.0 PR can be FWP up to master.

Forward-Port-Of: #19838

[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