[Prepack] Documentation on Molecule Chains

Improved the documentation on how chains are handled in the prepacker.
The prepacker segments chains into multiple molecules (where each
molecule can fit into a single cluster). The prepacker maintains
MoleculeChainIds to the chain these molecules are a part of. This allows
the cluster legalizer to easily put the molecules into clusters and
understand when a molecule is part of a special chain (for gain
calculation).

Author: AlexandreSinger

vpr/src/pack/prepack.h

@@ -56,9 +56,15 @@ enum class e_pack_pattern_molecule_type : bool {
  * @brief Represents a grouping of atom blocks that match a pack_pattern,
  *        these groups are intended to be placed as a single unit during packing
  *
- * A chain is a special type of pack pattern.  A chain can extend across multiple logic blocks.
- * Must segment the chain to fit in a logic block by identifying the actual atom that forms the root of the new chain.
- * Assumes that the root of a chain is the primitive that starts the chain or is driven from outside the logic block
+ * A chain is a special type of pack pattern since it can extend across multiple
+ * logic blocks. The prepacker segments the chain into molecules that each fit
+ * in a logic block by identifying the atom that forms the root of the chain,
+ * and starting the first molecule from it. Long chains can lead to multiple
+ * molecules; a new molecule is created as the chain is traversed every time we
+ * exceed the maximum number of bits a single logic block can implement. The
+ * MoleculeChainId can be used to identify the molecules that are part of the
+ * same chain; it is set to Invalid if a given molecule did not come from a
+ * chain.
  */
 class t_pack_molecule {
   public: