Improve encode documentation (#65)

Improve the documentation for QCBOREncode_Init() and QCBOREncode_Finish().

replace all use of the deprecated macro MakeUsefulBufOnStack().

Address #62 

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/test/qcbor_decode_tests.c b/test/qcbor_decode_tests.c
index 0327f2b..f748104 100644
--- a/test/qcbor_decode_tests.c
+++ b/test/qcbor_decode_tests.c
@@ -2113,7 +2113,7 @@
     through the use of SIZE_MAX.
    */
 
-   MakeUsefulBufOnStack(  HeadBuf, QCBOR_HEAD_BUFFER_SIZE);
+   UsefulBuf_MAKE_STACK_UB(  HeadBuf, QCBOR_HEAD_BUFFER_SIZE);
    UsefulBufC             EncodedHead;
 
    // This makes a CBOR head with a text string that is very long
@@ -6317,7 +6317,7 @@
 
 int32_t EnterBstrTest()
 {
-   MakeUsefulBufOnStack(OutputBuffer, 100);
+   UsefulBuf_MAKE_STACK_UB(OutputBuffer, 100);
 
    QCBORDecodeContext DC;
 
@@ -6752,7 +6752,7 @@
                     UsefulBuf_FROM_BYTE_ARRAY_LITERAL(spMapWithIndefLenStrings),
                     QCBOR_DECODE_MODE_NORMAL);
 
-   MakeUsefulBufOnStack(StringBuf, 200);
+   UsefulBuf_MAKE_STACK_UB(StringBuf, 200);
    QCBORDecode_SetMemPool(&DCtx, StringBuf, false);
 
    UsefulBufC ByteString;
diff --git a/test/qcbor_encode_tests.c b/test/qcbor_encode_tests.c
index 154bde7..b77ba40 100644
--- a/test/qcbor_encode_tests.c
+++ b/test/qcbor_encode_tests.c
@@ -2716,7 +2716,7 @@
     * other test exercises QCBOREncode_EncodeHead().
     */
    // ---- basic test to encode a zero ----
-   MakeUsefulBufOnStack(RightSize, QCBOR_HEAD_BUFFER_SIZE);
+   UsefulBuf_MAKE_STACK_UB(RightSize, QCBOR_HEAD_BUFFER_SIZE);
 
    UsefulBufC encoded = QCBOREncode_EncodeHead(RightSize,
                                                CBOR_MAJOR_TYPE_POSITIVE_INT,
@@ -2744,7 +2744,7 @@
 
 
    // ---- Try to encode into too-small a buffer ----
-   MakeUsefulBufOnStack(TooSmall, QCBOR_HEAD_BUFFER_SIZE-1);
+   UsefulBuf_MAKE_STACK_UB(TooSmall, QCBOR_HEAD_BUFFER_SIZE-1);
 
    encoded = QCBOREncode_EncodeHead(TooSmall,
                                     CBOR_MAJOR_TYPE_POSITIVE_INT,