ref-manual: add ZSTD_THREADS

ZSTD_THREADS is new for kirkstone.

(From yocto-docs rev: b970936b89bebb184c82be3a9514a9eba7c04f9b)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -8779,3 +8779,16 @@ system and gives an overview of their function and contents.
       memory and become problematic with parallelism elsewhere in the build.
       "50%" has been found to be a good value.
 
+   :term:`ZSTD_THREADS`
+      Specifies the number of parallel threads that should be used when
+      using ZStandard compression.
+
+      By default this scales with core count, but is never set less than 2
+      to ensure that multi-threaded mode is always used so that the output
+      file contents are deterministic. Builds will work with a value of 1
+      but the output will differ compared to the output from the compression
+      generated when more than one thread is used.
+
+      On systems where many tasks run in parallel, setting a limit to this
+      can be helpful in controlling system resource usage.
+