hw: explain misleading "Quota exceeded"

Upgrading the quota of a PD session on HW always triggers a "Quota
exceeded" warning. To prevent unecessary debugging effort in the future,
we explain in an in-code comment that the warning is normal.

Ref #1805

repos/base-hw/src/core/platform_pd.cc

@@ -114,6 +114,12 @@ void Capability_space::upgrade_slab(Allocator &alloc)
 {
 	for (;;) {
 		Slab_block * block;
+
+		/*
+		 * On every upgrade we try allocating as many blocks as possible.
+		 * If the underlying allocator complains that its quota is exceeded
+		 * this is normal as we use it as indication when to exit the loop.
+		 */
 		if (!alloc.alloc(SLAB_SIZE, &block)) return;
 		block = construct_at<Slab_block>(block, &_slab);
 		_slab.insert_sb(block);