[SwiftSyntax] Enhance clarity in the conditions for allocating a custom slab

Author: pinkjuice66

Sources/SwiftSyntax/BumpPtrAllocator.swift

@@ -111,8 +111,9 @@ public class BumpPtrAllocator {
       return allocated
     }
 
-    // If the size is too big, allocate a dedicated slab for it.
-    if byteCount >= self.initialSlabSize {
+    // If the size is too large to be contained in the new slab,
+    // allocate a dedicated slab for it.
+    if byteCount > self.slabSize(at: slabs.count) {
       let customSlab = Slab.allocate(
         byteCount: byteCount,
         alignment: alignment