docs/DSP: Document the behavior when main and extended opcodes both write to the same register (the write backlog)

For more information, ApplyWriteBackLog, WriteToBackLog, and ZeroWriteBackLog were added in b787f5f8f7 and the explanatory comment was added in fd40513fed, although it did not mention the specific instructions that could trigger this edge case. The statements about which registers can be written by main opcodes and extension opcodes are based on my own checking of all instructions in the manual.

docs/DSP/GameCube_DSP_Users_Manual/GameCube_DSP_Users_Manual.tex

@@ -4339,9 +4339,16 @@ Extended opcodes do not exist on their own. These opcodes can only be attached t
 Specifically, opcodes where the first nybble is 0, 1, or 2 cannot be extended.
 Opcodes where the first nybble is 4 or higher can be extended, using the 8 lower bits.
 Opcodes where the first nybble is 3 can also be extended, but the main opcode is 9 bits and the extension opcode is 7 bits.  For these instructions, the extension opcode is treated as if the first bit were 0 (i.e. \texttt{0xxxxxxx}).
+(\Opcode{NX} has no behavior of its own, so it can be used to get an extended opcode's behavior on its own.)
 
 Extended opcodes do not modify the program counter (\Register{\$pc} register).
 
+Extended opcodes are run \textit{in parallel} with the main opcode; they see the same register state as the input. (For instance, \texttt{\Opcode{MOVR}\Opcode{'MV} \Register{\$ac1}, \Register{\$ax0.l} : \Register{\$ax0.l}, \Register{\$ac1.m}} (encoded as \Value{0x6113}) \textit{swaps} the values of \Register{\$ac1.m} and \Register{\$ax0.l} (and also extends the new value of \Register{\$ac1.m} into \Register{\$ac1.l} and \Register{\$ac1.h}).)
+
+Since they are executed in parallel, the main and extension opcodes could theoretically write to the same registers. All opcodes that support extension only modify a main accumulator \Register{\$acD}, as well as \Register{\$prod}, \Register{\$sr}, and/or \Register{\$pc}, while the extension opcodes themselves generally only modify an additional accumulator \Register{\$axD} and addressing registers \Register{\$arS}. The exception is \Opcode{'L} and \Opcode{'LN}, which has the option of writing to \Register{\$acD}. Thus, \texttt{\Opcode{INC}\Opcode{'L} \Register{\$ac0} : \Register{\$ac0.l}, @\Register{\$ar0}} (encoded as \Value{0x7660}) increments \Register{\$ac0} (and thus \Register{\$ac0.l}), but also sets \Register{\$ac0.l} to the value in data memory at address \Register{\$ar0} and increments \Register{\$ar0}.
+
+When the main and extension opcodes write to the same register, the register is set to the two values bitwise-or'd together. For the above example, \Register{\$ar0.l} would be set to \InlineExpression{(\Register{\$ar0.l} + 1) | MEM[\Register{\$ar0}]}. \textbf{Note that no official uCode writes to the same register twice like this.}
+
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 \section{Alphabetical list of extended opcodes}