Document `stream_compress_adapt` has co-requisites to function

Document `zstd`-specific `stream_compress_adapt` requires the following to work:
- `stream_compress_long` active (see #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.
```

Search the repository for `stream_compress_adapt`, `stream_compress_long`, and `stream_compress_threads` to find the relevant documentation entry. Confirm the documented prerequisites and zstd limitations against the issue text and `man zstd`; the work is done when users can see the required co-requisites and incompatible mode.

Written by the indexing model from the issue text.