[ssi_af_ls] Adds an explanatory comment

xroot_plugins/XrdCtaArchiveFileLs.hpp

@@ -111,6 +111,11 @@ public:
                tf->set_f_seq(jt->second.fSeq);
                tf->set_block_id(jt->second.blockId);
 
+               // is_buffer_full is set to true when we have one full block of data in the buffer, i.e.
+               // enough data to send to the client. The actual buffer size is double the block size,
+               // so we can keep writing a few additional records after is_buffer_full is true. These
+               // will be sent on the next iteration. If we exceed the hard limit of double the block
+               // size, Push() will throw an exception.
                is_buffer_full = streambuf->Push(record);
             }
          }