docs: move section on untrusted files, cleanup [skip-ci]

Author: randy408

docs/decode.md

@@ -124,13 +124,27 @@ Moreover the size calculated by `spng_decoded_image_size()` can be checked
 against a hard limit before allocating memory for the output image.
 
 Chunks of arbitrary length (e.g. text, color profiles) take up additional memory,
-`spng_set_chunk_limits()` is used to set hard limits on chunk length- and cache limits,
-note that reaching either limit is handled as a fatal error.
+`spng_set_chunk_limits()` is used to set hard limits on chunk length and overall memory usage.
 
 Since v0.7.0 the `SPNG_CHUNK_COUNT_LIMIT` option controls how many chunks can be stored,
 the default is `1000` and is configurable through [`spng_set_option()`](context.md#spng_set_option),
 this limit is independent of the chunk cache limit.
 
+Note that exceeding any of the chunk limits is handled as an out-of-memory error.
+
+## Decoding untrusted files
+
+To decode untrusted files safely it is required to at least:
+
+* Set an upper limit on image dimensions with `spng_set_image_limits()`.
+
+* Use `spng_decoded_image_size()` to calculate the output image size
+ and check it against a constant limit.
+
+* Set a chunk size and chunk cache limit with `spng_set_chunks_limits()`
+  to control memory usage and avoid DoS from decompression bombs.
+
+
 # API
 
 See also: [spng_set_png_stream()](context.md#spng_set_png_stream), [spng_set_png_file()](context.md#spng_set_png_file).

docs/usage.md

@@ -26,19 +26,5 @@ spng_ctx_free(ctx);
 
 ```
 
-For a complete example see [example.c](https://github.com/randy408/libspng/blob/v0.7.3/examples/example.c).
-
-
-## Decoding untrusted files
-
-To decode untrusted files safely it is required to at least:
-
-* Set an upper image width and height limit with `spng_set_image_limits()`.
-
-* Use `spng_decoded_image_size()` to calculate the output image size
- and check it against a constant limit.
-
-* Set a chunk size and chunk cache limit with `spng_set_chunks_limits()`
-  to avoid running out of memory. Note that exceeding either limit is
-  handled as an out-of-memory error since v0.6.0.
-
+For a complete example see [example.c](https://github.com/randy408/libspng/blob/v0.7.3/examples/example.c)
+and [Decoding untrusted files](decode.md#decoding-untrusted-files)