docs: document the variable

Author: mdelapenya

modules/mssql/Makefile

@@ -1,5 +1,9 @@
 include ../../commons-test.mk
 
+# Enforce x509negativeserial=1. This is required in case the tests
+# use certificates with negative serial numbers.
+# Go 1.23+ rejects these certificates without this flag.
+# Cf https://pkg.go.dev/crypto/x509#ParseCertificate
 export GODEBUG=x509negativeserial=1
 
 .PHONY: test