specs: Clarify cannon vm syscall behavior

Makefile

@@ -130,4 +130,6 @@ update-op-geth:
 .PHONY: update-op-geth
 
 bedrock-markdown-links:
-	docker run --init -it -v `pwd`:/input lycheeverse/lychee --verbose --no-progress --exclude-loopback --exclude twitter.com --exclude explorer.optimism.io --exclude-mail /input/README.md "/input/specs/**/*.md"
+	docker run --init -it -v `pwd`:/input lycheeverse/lychee --verbose --no-progress --exclude-loopback \
+		--exclude twitter.com --exclude explorer.optimism.io --exclude linux-mips.org \
+		--exclude-mail /input/README.md "/input/specs/**/*.md"

specs/cannon-fault-proof-vm.md

@@ -106,7 +106,7 @@ The following table list summarizes the supported syscalls and their behaviors.
 | 4055 | fcntl | uint32 fd | int32 cmd | | Similar behavior as Linux/MIPS. Only the `F_GETFL` (3) cmd is supported. Sets errno to `0x16` for all other commands |
 
 For all of the above syscalls, an error is indicated by setting the return register (`$v0`) to
-`0xFFFFFFFF` (-1) and `errno` (`$a3`) is set accordingly.
+`0xFFFFFFFF` (-1) and `errno` (`$a3`) is set accordingly. The VM must not change the value of registers other than `$v0` and `$a3` during syscall handling.
 For unsupported syscalls, the VM must do nothing except to zero out the syscall return (`$v0`)
 and errno (`$a3`) registers.