Small update for filename in `isle_integration.md` (#5516)

* Small update for filename in `isle_integration.md`

The name "clif.isle" is stale (since #4953), now two files "clif_lower.isle" and "clif_opt.isle" are generated. Not sure if that PR necessitates other changes this this doc.

* Update cranelift/docs/isle-integration.md

Co-authored-by: Jamey Sharp <jsharp@fastly.com>

cranelift/docs/isle-integration.md

@@ -45,7 +45,7 @@ This will give pretty-printed errors with source context.
 Additionally, the `cranelift-codegen-meta` crate will automatically generate
 ISLE `extern` declarations and helpers for working with CLIF. The code that does
 this is defined inside `cranelift/codegen/meta/src/gen_inst.rs` and it creates
-the `clif.isle` file in the `target/` output directory, which is subsequently
+several ISLE files in the `target/` output directory which are subsequently
 read by the ISLE compiler as part of its prologue.
 
 ## Where are the relevant files?
@@ -55,9 +55,13 @@ read by the ISLE compiler as part of its prologue.
 * `cranelift/codegen/src/prelude.isle`: Common definitions and declarations for
   ISLE. This gets included in every ISLE compilation.
 
-* `target/.../out/clif.isle`: Auto-generated declarations and helpers for
-  working with CLIF inside ISLE. Generated by `cranelift/codegen/build.rs`.
-  This gets included in every ISLE compilation.
+* `target/.../out/clif_lower.isle`: Auto-generated declarations and helpers
+  for working with CLIF for instruction lowering inside ISLE. Generated by
+  `cranelift/codegen/build.rs`, which builds it into every backend.
+  
+* `target/.../out/clif_opt.isle`: Auto-generated declarations and helpers for
+  working with CLIF for mid-end optimizations. Generated by
+  `cranelift/codegen/build.rs`, which builds it into the mid-end optimizer.
 
 * `cranelift/codegen/src/machinst/isle.rs`: Common Rust code for gluing
   ISLE-generated code into a target architecture's backend. Contains