added unit tests for compressBound()

and rephrased the code documentation, as suggested by @terrelln

lib/zstd.h

@@ -204,11 +204,10 @@ ZSTDLIB_API size_t ZSTD_findFrameCompressedSize(const void* src, size_t srcSize)
 /* ZSTD_compressBound() :
  * maximum compressed size in worst case single-pass scenario.
  * When invoking `ZSTD_compress()` or any other one-pass compression function,
- * providing @dstCapacity >= ZSTD_compressBound(srcSize) guarantees success.
- * Note that it's still allowed to provide a smaller @dstCapacity value,
- * in which case, the caller must inspect the return value with ZSTD_isError(),
- * to detect any potential compression failure.
- * Note : ZSTD_compressBound() itself can fail, if @srcSize is too large.
+ * it's recommended to provide @dstCapacity >= ZSTD_compressBound(srcSize)
+ * as it eliminates one potential failure scenario,
+ * aka not enough room in dst buffer to write the compressed frame.
+ * Note : ZSTD_compressBound() itself can fail, if @srcSize > ZSTD_MAX_INPUT_SIZE .
  *        In which case, ZSTD_compressBound() will return an error code
  *        which can be tested using ZSTD_isError().
  *
@@ -219,7 +218,7 @@ ZSTDLIB_API size_t ZSTD_findFrameCompressedSize(const void* src, size_t srcSize)
  * Will produce constant value 0 if srcSize too large.
  */
 #define ZSTD_MAX_INPUT_SIZE ((sizeof(size_t)==8) ? 0xFF00FF00FF00FF00LLU : 0xFF00FF00U)
-#define ZSTD_COMPRESSBOUND(srcSize)   (((unsigned long long)(srcSize) > ZSTD_MAX_INPUT_SIZE) ? 0 : (srcSize) + ((srcSize)>>8) + (((srcSize) < (128<<10)) ? (((128<<10) - (srcSize)) >> 11) /* margin, from 64 to 0 */ : 0))  /* this formula ensures that bound(A) + bound(B) <= bound(A+B) as long as A and B >= 128 KB */
+#define ZSTD_COMPRESSBOUND(srcSize)   (((size_t)(srcSize) > ZSTD_MAX_INPUT_SIZE) ? 0 : (srcSize) + ((srcSize)>>8) + (((srcSize) < (128<<10)) ? (((128<<10) - (srcSize)) >> 11) /* margin, from 64 to 0 */ : 0))  /* this formula ensures that bound(A) + bound(B) <= bound(A+B) as long as A and B >= 128 KB */
 ZSTDLIB_API size_t ZSTD_compressBound(size_t srcSize); /*!< maximum compressed size in worst case single-pass scenario */
 /* ZSTD_isError() :
  * Most ZSTD_* functions returning a size_t value can be tested for error,