Hopefully improve commenting of #5126.

Reading the PR gave me the opportunity to better specify what the code
was doing in places where I was not immediately sure about what was
going on. Moreover I documented the structure in server.h so that people
reading the header file will immediately understand what the structure
is useful for.

src/server.h

@@ -619,6 +619,8 @@ typedef struct redisObject {
 
 struct evictionPoolEntry; /* Defined in evict.c */
 
+/* This structure is used in order to represent the output buffer of a client,
+ * which is actually a linked list of blocks like that, that is: client->reply. */
 typedef struct clientReplyBlock {
     size_t size, used;
     char buf[];