Documented output phase

phases/output.mli

@@ -1 +1,14 @@
+(** Save assembly to output file or print to [stdout]. *)
+
+(** If an output file is specified in the [-o] command-line argument, the
+    genrated assembly code is save to that file. Otherwise, the final assembly
+    code is printed to [stdout]. This allows for using the compiler with UNIX
+    pipes, since other output is written to [stderr] and the {!Load} phase can
+    also read from [stdin]:
+    {v ./civicc < foo.cvc > foo.s v}
+
+    This phase has [Assembly] as input and outputs [Empty].
+    *)
+
+(** Main phase function, called by {!Main}. *)
 val phase : Main.phase_func