Document `stream_compress_adapt` has co-requisites to function

[HaleTom]

Document zstd-specific stream_compress_adapt requires the following to work:

• stream_compress_long active (see stream_compress_long default doesn't add --long to command line #615)
• stream_compress_threads (set to 0 for zstd auto-detect CPU cores)

man zstd says:

     •   --adapt[=min=#,max=#]: zstd will dynamically  adapt  compression  level  to
         perceived I/O conditions. Compression level adaptation can be observed live
         by using command -v. Adaptation can be constrained between supplied min and
         max levels. The feature works when combined with multi-threading and --long
         mode.  It  does not work with --single-thread. It sets window size to 8 MiB
         by default (can be changed manually, see wlog). Due to the  chaotic  nature
         of dynamic adaptation, compressed result is not reproducible.