arch/x86/include/smm_call: improve documentation of call_smm

Since the inline assembly code in call_smm doesn't make it exactly
obvious how this function to call the APMC SMI handler works in detail,
add a more detailed explanation as comment.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I3566af191492ce00a3033335ff80e01c33e98e63
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79834
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>

src/arch/x86/include/smm_call.h

@@ -4,7 +4,10 @@
 #include <cpu/x86/smm.h>
 
 /*
- * calls into SMM with the given cmd and subcmd in eax, and arg in ebx
+ * Call the APMC SMI handler that resides in SMM. First, the command and sub-command are stored
+ * in eax, and the argument pointer is stored in ebx, then the command byte is written to the
+ * APMC IO port to trigger the SMI. The APMC SMI handler then reads the command from the APMC
+ * IO port and the contents of eax and ebx from the SMM state save area.
  *
  * static inline because the resulting assembly is often smaller than
  * the call sequence due to constant folding.