Add documentation comment for TiDB versions in Makefile

Document that TiDB versions in Makefile acceptance target must match
the TIDB_VERSIONS in .github/workflows/main.yml. All versions are
already correctly set to latest of each minor series.

Author: zph

GNUmakefile

@@ -49,6 +49,8 @@ bin/terraform:
 testacc: fmtcheck bin/terraform
 	PATH="$(CURDIR)/bin:${PATH}" TF_ACC=1 go test $(TEST) -v $(TESTARGS) -timeout=90s
 
+# TiDB versions: latest of each minor series (must match .github/workflows/main.yml TIDB_VERSIONS)
+# 6.1.x → 6.1.7, 6.5.x → 6.5.12, 7.1.x → 7.1.6, 7.5.x → 7.5.7, 8.1.x → 8.1.2, 8.5.x → 8.5.3
 acceptance: testversion5.6 testversion5.7 testversion8.0 testpercona5.7 testpercona8.0 testmariadb10.3 testmariadb10.8 testmariadb10.10 testtidb6.1.7 testtidb6.5.12 testtidb7.1.6 testtidb7.5.7 testtidb8.1.2 testtidb8.5.3
 
 testversion%: