static dctx is incompatible with legacy support

documented, and runtime tested
diff --git a/doc/zstd_manual.html b/doc/zstd_manual.html
index a4b59bc..156e7dd 100644
--- a/doc/zstd_manual.html
+++ b/doc/zstd_manual.html
@@ -718,7 +718,8 @@
  @return : pointer to ZSTD_DCtx*, or NULL if error (size too small)
   Note : zstd will never resize nor malloc() when using a static dctx.
          If it needs more memory than available, it will simply error out.
-  Note 2 : there is no corresponding "free" function.
+  Note 2 : static dctx is incompatible with legacy support
+  Note 3 : there is no corresponding "free" function.
            Since workspace was allocated externally, it must be freed externally.
   Limitation : currently not compatible with internal DDict creation,
                such as ZSTD_initDStream_usingDict().