docs: expand thread safety documentation to cover derived values

Closes #166.

docs/api_usage.rst

@@ -97,11 +97,22 @@ about thread safety. Do not operate on the same ``ZstdCompressor`` and
 fine to have different threads call into a single instance, just not at the
 same time.
 
+Objects derived from ``ZstdCompressor`` and ``ZstdDecompressor`` that
+perform (de)compression operations (such as ``ZstdCompressionReader`` and
+``ZstdDecompressionWriter``) are bound to the ``ZstdCompressor`` or
+``ZstdDecompressor`` from which they came and are therefore not thread safe
+by extension.
+
 Some operations require multiple function calls to complete. e.g. streaming
 operations. A single ``ZstdCompressor`` or ``ZstdDecompressor`` cannot be used
 for simultaneously active operations. e.g. you must not start a streaming
 operation when another streaming operation is already active.
 
+If you need to perform multiple compression or decompression operations in
+parallel, you MUST construct multiple ``ZstdCompressor`` or ``ZstdDecompressor``
+instances so each independent operation has its own ``ZstdCompressor`` or
+``ZstdDecompressor`` instance.
+
 The C extension releases the GIL during non-trivial calls into the zstd C
 API. Non-trivial calls are notably compression and decompression. Trivial
 calls are things like parsing frame parameters. Where the GIL is released

docs/news.rst

@@ -110,6 +110,9 @@ Changes
   backend. (#165)
 * ``ZstdCompressionWriter`` and ``ZstdDecompressionWriter`` now implement
   ``__iter__`` and ``__next__``, which always raise ``io.UnsupportedOperation``.
+* Documentation on thread safety has been updated to note that derived objects
+  like ``ZstdCompressionWriter`` have the same thread unsafety as the contexts
+  they were derived from. (#166)
 
 0.16.0 (released 2021-10-16)
 ============================