document the directive

Author: tshepang

src/doc/rustc-dev-guide/src/tests/directives.md

@@ -192,6 +192,8 @@ settings:
   specified atomic widths, e.g. the test with `//@ needs-target-has-atomic: 8,
   16, ptr` will only run if it supports the comma-separated list of atomic
   widths.
+- `needs-dynamic-linking` - ignores if target does not support dynamic linking
+  (which is orthogonal to it being unable to create `dylib` and `cdylib` crate types)
 
 The following directives will check LLVM support: