spec update : require minimum nb of literals for 4-streams mode

Reported by @shulib :
the specification for 4-streams mode
doesn't work when the amount of literals to compress is 5 bytes.
Extending it, it also doesn't work for sizes 1 or 2.

This patch updates the specification and the implementation
to require a minimum of 6 literals to trigger or accept the 4-streams mode.

The impact is expected to be a no-op :
the 4-streams mode is never triggered for such small quantity of literals anyway,
since it would be wasteful (it costs ~7.3 bytes more than single-stream mode).
An informal lower limit is set at ~256 bytes,
so the technical minimum is very far from this limit.

This is just meant for completeness of the specification.

lib/common/huf.h

@@ -88,8 +88,10 @@ HUF_PUBLIC_API size_t HUF_compress2 (void* dst, size_t dstCapacity,
                                unsigned maxSymbolValue, unsigned tableLog);
 
 /** HUF_compress4X_wksp() :
- *  Same as HUF_compress2(), but uses externally allocated `workSpace`.
- * `workspace` must be at least as large as HUF_WORKSPACE_SIZE */
+ *  Same as HUF_compress2(), but uses externally allocated @workSpace.
+ * @workSpace's size, aka @wkspSize, must be >= HUF_WORKSPACE_SIZE
+ * @srcSize must be >= 6
+ */
 #define HUF_WORKSPACE_SIZE ((8 << 10) + 512 /* sorting scratch space */)
 #define HUF_WORKSPACE_SIZE_U64 (HUF_WORKSPACE_SIZE / sizeof(U64))
 HUF_PUBLIC_API size_t HUF_compress4X_wksp (void* dst, size_t dstCapacity,