Docs: refresh markdown

docs/api-output.md

@@ -423,6 +423,7 @@ Warning: multiple sharp instances concurrently producing tile output can expose
     -   `options.layout` **[string][2]** filesystem layout, possible values are `dz`, `iiif`, `zoomify` or `google`. (optional, default `'dz'`)
     -   `options.centre` **[boolean][7]** centre image in tile. (optional, default `false`)
     -   `options.center` **[boolean][7]** alternative spelling of centre. (optional, default `false`)
+    -   `options.id` **[string][2]** when `layout` is `iiif`, sets the `@id` attribute of `info.json` (optional, default `'https://example.com/iiif'`)
 
 ### Examples
 

docs/api-utility.md

@@ -84,8 +84,12 @@ Returns **[Object][1]**
 
 Gets or, when a concurrency is provided, sets
 the number of threads _libvips'_ should create to process each image.
-The default value is the number of CPU cores.
-A value of `0` will reset to this default.
+
+The default value is the number of CPU cores,
+except when using glibc-based Linux without jemalloc,
+where the default is `1` to help reduce memory fragmentation.
+
+A value of `0` will reset this to the number of CPU cores.
 
 The maximum number of images that can be processed in parallel
 is limited by libuv's `UV_THREADPOOL_SIZE` environment variable.