Move build system documentation

Bug: 62201421

Test: make help
Change-Id: I7d7b448d3f736e171ebfbdceee46eef7de748742

ui/build/build.go

@@ -102,8 +102,7 @@ func checkCaseSensitivity(ctx Context, config Config) {
 }
 
 func help(ctx Context, config Config, what int) {
-	cmd := Command(ctx, config, "make",
-		"make", "-f", "build/core/help.mk")
+	cmd := Command(ctx, config, "help.sh", "build/make/help.sh")
 	cmd.Sandbox = makeSandbox
 	cmd.Stdout = ctx.Stdout()
 	cmd.Stderr = ctx.Stderr()