[ADD] accounting: document tax excl. / tax incl. toggle

[dade-odoo]

taskid-6344726

Adds an explanation of how the new Tax Excl. / Tax Incl. toggle on quotes/SOs/POs/invoices works.
Also adds more context to the tax computation page in general to make the page more clear.

[robodoo]

[xpl-odoo]

Thanks for the PR @dade-odoo !
Already delegating, even though I got a suggestion for you regarding the way we present the info about the toggle (feel free to use/adapt as you see fit)

@robodoo delegate+

[xpl-odoo]

What do you think about putting all the info about the toggle inside an alert block (could be a note instead, or even a sub-heading?)
I gave it a try, and I think it's easier to follow that way. I didn't include your note about using the toggle as I felt it wasn't necessary here. I put the rst just in case you want to go for it, but feel free to rephrase/adapt as you see fit.

:guilabel:`Default` indicates that the tax follows the company-wide default.

:guilabel:`Tax Excluded` indicates that the tax amount is not included in the sales price. The tax
computation will therefore compute a tax amount on top of the sales price.

:guilabel:`Tax Included` indicates that the tax amount is included in the sales price. The tax
computation will therefore split the sales price into a tax-excluded base and the tax amount. This
makes it suitable for B2C sales in most countries, where prices are quoted tax-inclusive.

.. important::
   If the :guilabel:`Default` option is selected, using the :guilabel:`Tax Excl. / Tax Incl.` toggle
   on a specific quotation, sales order, purchase order, or invoice changes this tax's behavior for
   that record only.

   However, if a tax's :guilabel:`Included in Price` field is set to either :guilabel:`Tax Excluded`
   or :guilabel:`Tax Included`, the tax's behavior will not be changed by the :guilabel:`Tax Excl. /
   Tax Incl.` toggle on quotations, sales orders, purchase orders, and invoices.

[dade-odoo]

Thanks for this recommendation - I do like the alert block idea, but I feel like that leaves out a little bit of context that would be nice to add (both an intro sentence explaining the use case and the note I had explaining that you should add products before using the toggle). I've tried a sub-section as you suggested, which I like, but it does mean that we have a single H3. I prefer the sub-section but I'm open to suggestions if you prefer the alert block! Let me know :)

[xpl-odoo]

The info about the toggle felt a bit out of place with the rest of the page so that's why I wasn't sure about having a separate section, but it's clearer that way so go for it :)

[dade-odoo]

@robodoo r+