Add some more docs

crates/wasmtime/Cargo.toml

@@ -301,6 +301,13 @@ std = [
   'wasmtime-environ/std',
   'object/std',
   'once_cell',
+  # technically this isn't necessary but once you have the standard library you
+  # probably want things to go fast in which case you've probably got signal
+  # handlers and such so implicitly enable this. This also helps reduce the
+  # verbosity of others depending on `wasmtime` with `default-features = false`
+  # where frequently `std` is enabled and this feature will typically want to be
+  # enabled by default as well.
+  'signals-based-traps',
 ]
 
 # Enables support for the `Store::call_hook` API which enables injecting custom

crates/wasmtime/src/lib.rs

@@ -266,6 +266,12 @@
 //!   with the same overhead as the `call-hook` feature where entries/exits into
 //!   WebAssembly will have more overhead than before.
 //!
+//! * `signals-based-traps` - Enabled by default, this enables support for using
+//!   host signal handlers to implement WebAssembly traps. For example virtual
+//!   memory is used to catch out-of-bounds accesses in WebAssembly that result
+//!   in segfaults. This is implicitly enabled by the `std` feature and is the
+//!   best way to get high-performance WebAssembly.
+//!
 //! More crate features can be found in the [manifest] of Wasmtime itself for
 //! seeing what can be enabled and disabled.
 //!