Microsoft Excel: How to Comment Cells

I came to the industry at the end of that times when the computer program executes as it was written. I saw a quite big machines, occupied more than 100 square meters for its central processor, but I started my professional activity as a programmer on IBM PC clones. There were different CPU architectures (68k, PowerPC, 386/486, SPARC...) when I began, but Intel defeated them all with Pentium CPU (marketing) family.
I saw the knowledge and technology fragmentation. I saw many technologies started, developed and retired. However, I have not seen many things yet.
I have some experience, but my experience is not perfectly comprehensive. I still have many things to learn and I still cannot make a poker face when I find out some aspects about how the things were designed and works. My experience does not make me an absolute expert in some areas, because these areas are changing. I know some things, but I also understand that some things I know could be useless for nowadays.