Clarify docs on writing

Author: alexcrichton

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -625,9 +625,9 @@ pub fn create_compressed_metadata_file_for_xcoff(
 /// that contains a custom section of the name `section_name` with contents
 /// `data`.
 ///
-/// NB: the wasm file format is simple enough that for now an extra crate from
-/// crates.io (such as `wasm-encoder`) isn't used at this time (nor `wasmparser`
-/// for example to parse). The file format is:
+/// NB: the `object` crate does not yet have support for writing the the wasm
+/// object file format. The format is simple enough that for now an extra crate
+/// from crates.io (such as `wasm-encoder`). The file format is:
 ///
 /// * 4-byte header "\0asm"
 /// * 4-byte version number - 1u32 in little-endian format