Add note about libdispatch docs

Author: ethanc8

source/APIRefs/Dispatch/index.md

@@ -5,3 +5,7 @@
 Comprehensive support for concurrent code execution on multicore hardware.
 
 * <a href="HeaderDoc/masterTOC.html">**Objective-C API documentation**</a>
+
+:::{note}
+The API documentation was built from [libdispatch for Swift 6.2, with patches applied to make the documentation work](https://github.com/ethanc8/swift-corelibs-libdispatch/tree/documentation) using [a slightly patched HeaderDoc](https://github.com/ethanc8/headerdoc/tree/gnustep). HeaderDoc must be installed to `/usr/local` and not inside a Conda environment (as far as I know) since Conda's packaging for perl seems to not work with HeaderDoc.
+:::