docs(core): quantity is not a consumable's wire amount

FIFA 17's consumable `amount` is the definition's effect magnitude (a "+15
training" card), measured: across every owned consumable in the real profile
the observed `amount` equals its `fcc_*` table row, and each copy is its own
instance carrying the same value. Recording it as `quantity` would claim the
club owns fifteen of them.

src/services/import.rs

@@ -55,8 +55,13 @@ pub struct ImportOwnedCard {
     /// `cardsubtypeid`, resource ranges, …) onto this before calling Core.
     #[serde(default)]
     pub content_kind: ContentKind,
-    /// Optional per-instance stack size (a consumable's wire `amount`). Absent /
+    /// Optional per-instance stack size. Absent / `null` means "not a stack"; it
-    /// `null` means "not a stack"; it never collapses two instances into one row.
+    /// never collapses two instances into one row.
+    ///
+    /// NOT a consumable's wire `amount`: in FIFA 17 that field is the
+    /// definition's effect magnitude (a "+15 training" card), and every copy is
+    /// its own instance carrying the same value, so storing it here would claim
+    /// the club owns fifteen of them.
     #[serde(default)]
     pub quantity: Option<i64>,
 }