Update a bunch of comments from before wasi support was added

Author: bjorn3

std/src/sys/alloc/wasm.rs

@@ -1,6 +1,6 @@
 //! This is an implementation of a global allocator on wasm targets when
-//! emscripten is not in use. In that situation there's no actual runtime for us
-//! to lean on for allocation, so instead we provide our own!
+//! emscripten or wasi is not in use. In that situation there's no actual runtime
+//! for us to lean on for allocation, so instead we provide our own!
 //!
 //! The wasm instruction set has two instructions for getting the current
 //! amount of memory and growing the amount of memory. These instructions are the

std/src/sys/pal/wasi/mod.rs

@@ -1,8 +1,7 @@
 //! System bindings for the wasm/web platform
 //!
 //! This module contains the facade (aka platform-specific) implementations of
-//! OS level functionality for wasm. Note that this wasm is *not* the emscripten
-//! wasm, so we have no runtime here.
+//! OS level functionality for wasm.
 //!
 //! This is all super highly experimental and not actually intended for
 //! wide/production use yet, it's still all in the experimental category. This

std/src/sys/pal/wasm/mod.rs

@@ -2,7 +2,7 @@
 //!
 //! This module contains the facade (aka platform-specific) implementations of
 //! OS level functionality for wasm. Note that this wasm is *not* the emscripten
-//! wasm, so we have no runtime here.
+//! or wasi wasm, so we have no runtime here.
 //!
 //! This is all super highly experimental and not actually intended for
 //! wide/production use yet, it's still all in the experimental category. This