Say which container $this->get() resolves against in the front office

[boo-code]

Questions	Answers
Branch?	9.x
Description?	The Symfony Services section of modules/concepts/services describes config/services.yml and $this->get() with no indication that both only apply to the back office and to pages already migrated to Symfony. The front office caveat exists but sits under Services in Legacy environment, several screens further down, which is where a reader lands only after the call has already thrown. Adds a warning notice at the top of the section naming what resolves where, and linking to the existing section.
Fixed ticket?	Related to PrestaShop/PrestaShop#29013
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Why here

#29013 is a module author following the top of this page and getting
You have requested a non-existent service "twig" from a front office hook. Twenty-one comments and three
community members later, the answer was the paragraph further down the same page. The page is not wrong;
the scoping is stated too late to help.

The facts the notice states, measured on 9.2

A probe module hooked on displayFooterProduct, hit through a real front office product page:

$this->get('router')                    ServiceNotFoundException: non-existent service "router"
$this->get('twig')                      NULL           (9.0+ routes it to getTwig())
$this->get('translator')                ServiceNotFoundException
$this->get('prestashop.adapter.legacy.context')   PrestaShop\PrestaShop\Adapter\LegacyContext
$this->getTwig()                        NULL
SymfonyContainer::getInstance()         NULL
container class                         FrontContainer, 109 services

The 109 are Doctrine plus prestashop.adapter.* / prestashop.core.* entries; the only translation-ish
one is prestashop.translation.translator_language_loader. No count is quoted in the notice, since it
would go stale.