core/plugins/quota: warn that a block-scoped limit shadows userdb overrides

[rokokol]

The Per-User Quota examples override quota_storage_size from userdb, and the page's main example deliberately keeps that setting outside the quota { .. } block ("Keep this setting outside the quota { .. } to allow easily overriding it in userdb lookups"). What the page never states is what happens when the limit is declared inside the block: the block-scoped value shadows the flat userdb override, which is then silently ignored — while doveadm user, which this section recommends for verification, still echoes the field back as if it had taken effect.

Measured on stock 2.4.4 with a passwd-file userdb, both directions: userdb_quota_storage_size=100M overrides a top-level quota_storage_size = 1G and is ignored when the limit is declared inside a named block; userdb_quota/<name>/storage_size addresses the block-scoped limit.

This trips users migrating from 2.3, where the limit commonly lived inside plugin { quota_rule = ... } — see e.g. Quota plugin: "quota_storage_size" not recognized from userdb extra fields in dovecot 2.4.1 on the mailing list.

This adds a warning box to the Per-User Quota section naming the failure mode and the path form that addresses a block-scoped limit.