Improve instruction documentation

[nummacway]

STOP
When reading through gbz80(7)'s documentation, I noticed it says STOP was 2 bytes long, when the instruction itself is only 1, not located in CB and doesn't take useful arguments. People on Discord pointed me to this chart (which BTW misses a "Yes" label at the very bottom). I suggest the documentation say something like this:
"Because the CPU might take the next byte as either a useless argument to the STOP instruction or as a new opcode, rgbds will always add an implicit NOP for safety. Visit Pan Docs for more information."

Some other suggestions:

• Legend:
  • Exclamation mark is only used once, and that's in the legend. There's no need to explain symbols that are never used.
  • u3 should say that it always refers to bit indizes, and if 0 refers to the least or most significant bit.
• Remove pointless anchors, e.g. any of these that aren't a heading. By using most browsers' default style of <abbr> HTML tag, one would expect the anchors to have a hint or point to the legend (or something like that), when in fact it's just a useless link. The legend of the 8-bit instructions looks really odd as B, D, E and L are underlined, but A, C, and H are not.
• CPL should mention that the operation is also known as "bitwise NOT". I do programming for 20 years (though in high-level languages) and this was the first time I ever saw this operation being called the complement.
• It could say that LD r8,r8 doesn't do anything at all if both arguments are the same. I thought that there must be a use if these exist, but LD [HL], [HL] does not - but there's no use, except for BGB using LD B, B for breakpoints and LD D, D for debug.
• Several commands that work with a constant (literal, value, immediate data) could state that more efficient shortcuts exist if that value is 0, e.g. LD A, 0 could be written as XOR A, and CP 0 could be written as AND A or OR A, although flags are different. I acknowledge that this document is probably the wrong place for notes like that. If so, is there a location for community notes and advice on individual instructions?