Document inferred const args

Author: BoxyUwU

src/items/generics.md

@@ -227,6 +227,22 @@ fn generic<const B: bool>() {
 }
 ```
 
+r[items.generics.const.inferred]
+
+r[items.generics.const.inferred.syntax]
+```grammar,types
+@root InferredConst -> 
+      `_`
+    | `(` InferredConst `)`
+```
+
+r[items.generics.const.inferred.intro]
+The inferred const asks the compiler to infer the const argument if possible
+based on the surrounding information available.
+
+r[items.generics.const.inferred.constraint]
+It cannot be used in item signatures.
+
 r[items.generics.where]
 ## Where clauses