Update documentation

Author: Kobzol

collector/compile-benchmarks/README.md

@@ -173,8 +173,9 @@ Rust code being written today.
     anything we should be aware of when using this crate as a compile-time
     benchmark.
   - Look at [crates.io](https://crates.io) to find the latest (non-prerelease) version.
-  - Download it with `collector download -c $CATEGORY crate $NAME $VERSION`.
-    The `$CATEGORY` is probably `primary`.
+  - Download it with `collector download -c $CATEGORY -a $ARTIFACT crate $NAME $VERSION`.
+    The `$CATEGORY` is probably `primary`. `$ARTIFACT` is either `library` or `binary`, depending
+    on what kind of artifact does the benchmark build.
 - It makes it easier for reviewers if you split things into two commits.
 - In the first commit, just add the code for the entire benchmark.
   - Do this by doing `git add` on the new directory.

docs/glossary.md

@@ -22,6 +22,7 @@ The following is a glossary of domain specific terminology. Although benchmarks
   - `incr-unchanged`: incremental compilation is used, with a full incremental cache and no code changes made.
   - `incr-patched`: incremental compilation is used, with a full incremental cache and some code changes made.
 * **category**: a high-level group of benchmarks. Currently, there are three categories, primary (mostly real-world crates), secondary (mostly stress tests), and stable (old real-world crates, only used for the dashboard).
+* **artifact type**: describes what kind of artifact does the benchmark build. Either `library` or `binary`.
 
 ### Types of compile-time benchmarks