Cleanups, add comments and explanations

lib/compress/zstd_compress_internal.h

@@ -94,7 +94,7 @@ typedef struct {
 
 typedef struct {
   rawSeq* seq;     /* The start of the sequences */
-  BYTE const* base;
+  BYTE const* base; /* The match state window base when LDMs were generated */
   size_t pos;      /* The position where reading stopped. <= size. */
   size_t size;     /* The number of sequences. <= capacity. */
   size_t capacity; /* The capacity starting from `seq` pointer */