Document amd64 platform-specific code

kernel/amd64/msr.c

@@ -1,12 +1,14 @@
 #include <amd64/msr.h>
 #include <libk/std.h>
 
+/// Read a model-specific register
 uint64_t amd64_rdmsr (uint32_t msr) {
   uint32_t low, high;
   __asm__ volatile ("rdmsr" : "=a"(low), "=d"(high) : "c"(msr));
   return ((uint64_t)high << 32 | (uint64_t)low);
 }
 
+/// Write a model-specific register
 void amd64_wrmsr (uint32_t msr, uint64_t value) {
   uint32_t low = (uint32_t)(value & 0xFFFFFFFF);
   uint32_t high = (uint32_t)(value >> 32);