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/error_private.c

@@ -29,6 +29,7 @@ const char* ERR_getErrorString(ERR_enum code)
     case PREFIX(frameParameter_windowTooLarge): return "Frame requires too much memory for decoding";
     case PREFIX(corruption_detected): return "Data corruption detected";
     case PREFIX(checksum_wrong): return "Restored data doesn't match checksum";
+    case PREFIX(literals_headerWrong): return "Header of Literals' block doesn't respect format specification";
     case PREFIX(parameter_unsupported): return "Unsupported parameter";
     case PREFIX(parameter_outOfBound): return "Parameter is out of bound";
     case PREFIX(init_missing): return "Context should be init first";