Quiet warnings for stricter compilers configurations (#84)

Fix warnings from -Wcast-qual and -Wstrict-prototypes.

A few other warning fixes and documentation improvements are added.

Also fixes Makefile so the before-release build script can invoke both gcc and clang. That script is here: laurencelundblade/qdv@536f083

This is an alternative to #79. It does not fix warnings for -Wbad-function-cast
because those fixes cause warnings with other compilers (MacOS clang). This doesn't enable GitHub automated build checks. The plan is to do that separately.
diff --git a/inc/qcbor/qcbor_encode.h b/inc/qcbor/qcbor_encode.h
index 2460897..884ddaa 100644
--- a/inc/qcbor/qcbor_encode.h
+++ b/inc/qcbor/qcbor_encode.h
@@ -1,6 +1,6 @@
 /*==============================================================================
  Copyright (c) 2016-2018, The Linux Foundation.
- Copyright (c) 2018-2020, Laurence Lundblade.
+ Copyright (c) 2018-2021, Laurence Lundblade.
  All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -1722,6 +1722,8 @@
  @param[out] pEncodedCBOR  Structure in which the pointer and length of the encoded
                            CBOR is returned.
 
+ @retval QCBOR_SUCCESS                     Encoded CBOR is returned.
+
  @retval QCBOR_ERR_TOO_MANY_CLOSES         Nesting error
 
  @retval QCBOR_ERR_CLOSE_MISMATCH          Nesting error