Add the `uffd` feature to the wasmtime crate docs.
//! * `vtune` - Not enabled by default, this feature compiles in support for
//! supporting VTune profiling of JIT code.
//!
+//! * `uffd` - Not enabled by default. This feature enables `userfaultfd` support
+//! when using the pooling instance allocator. As handling page faults in userspace
+//! comes with a performance penalty, this feature should only be enabled when kernel
+//! lock contention is hampering multithreading throughput. This feature is only
+//! supported on Linux and requires a Linux kernel version 4.11 or higher.
+//!
//! ## Examples
//!
//! In addition to the examples below be sure to check out the [online embedding
