Document three undocumented 5.0 behaviors (state, serialization, errors)

[soyuka]

Summary

Three undocumented API Platform 5.0-only behaviour changes, verified against api-platform/core at upstream/main:

• core/state-providers.md — a ParameterProviderInterface on a uriVariables Link can now replace the URI variable value used to fetch the resource (core #8491, commit fce2bdfe6). Documents the default transform-and-replace behaviour and the PreservesUriVariableInterface / write_uri_variable opt-out used by ReadLinkParameterProvider.
• core/serialization.md — for Doctrine JOINED/SINGLE_TABLE inheritance, a serializer group declared only on a discriminator subclass is now taken into account when deciding whether a relation is embedded (core #8283, commit a8af8e1b9). This is a behaviour change for existing APIs: relations that used to serialize as an IRI may now be embedded. Documents the rule and the readableLink/writableLink: false opt-out to keep the old behaviour.
• core/errors.md — the default exception_to_status mapping was undocumented and the shipped Symfony config sample had drifted from the source (wrong InvalidArgumentException namespace, a nonexistent ValidationExceptionInterface class). Reconciled against Configuration::addExceptionToStatusSection(): SerializerExceptionInterface => 400, InvalidArgumentException => 400, OptimisticLockException => 409, and the new UniqueConstraintViolationException => 422 (core #8478).

Each claim was checked directly against the shipped PHP source (see commit messages for file:line references), not copied from the audit prose.

Not fixed (out of scope, flagged for follow-up)

• core/errors.md's Laravel exception_to_status example still lists a mismatched default set (ExceptionInterface, InvalidArgumentException, phantom ValidationExceptionInterface) that doesn't match Laravel's actual shipped default (Illuminate\Auth\AuthenticationException => 401, Illuminate\Auth\Access\AuthorizationException => 403, see src/Laravel/config/api-platform.php:116-119). Left untouched — unrelated to the Doctrine/UniqueConstraintViolationException scope of this PR and would need its own verification pass.
• G7 (serialization.md) may overlap with open PR feat: support polymorphism #2278 ("support polymorphism") — worth checking before merge.

Test plan

• npx prettier@3.9.5 --check "**/*.md" --prose-wrap always — only pre-existing failures in core/doctrine-filters.md and core/filters.md remain (untouched by this PR).