linter: Correct text regarding benchmark script

I was mistaken about what the `config` flag does: the specified options
file is only used to exclude lint rules from the list of all
registered lint rules.

Change-Id: Iacaa7e283e0de33408b1d8aaa43932336a9ae86c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391565
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: srawlins

pkg/linter/CONTRIBUTING.md

@@ -88,22 +88,26 @@ Lint rules can be benchmarked with real code on disk. (There is no
 micro-benchmarking suite.) Use the `tool/benchmark.dart` script to execute one
 or more lint rules against a corpus of code. Here are some examples:
 
-Execute all of the lint rules enabled by a specified analysis options file
-against all of the code in the specified directory.
+Execute all known lint rules against all of the code in a specified
+directory.
 
 ```none
-dart tool/benchmark.dart \
-    --config $HOME/my/example/analysis_options.yaml \
-    $HOME/my/example/package
+dart tool/benchmark.dart $HOME/my/example/package
+```
+
+Execute all known lint rules against all of the code in the specified file.
+
+```none
+dart tool/benchmark.dart $HOME/my/example/package/lib/some_file.dart
 ```
 
-Execute all of the lint rules enabled by a specified analysis options file
-against all of the code in the specified file.
+Execute all known lint rules except those _explicitly disabled_ by a specified
+analysis options file against all of the code in the specified directory.
 
 ```none
 dart tool/benchmark.dart \
     --config $HOME/my/example/analysis_options.yaml \
-    $HOME/my/example/package/lib/some_file.dart
+    $HOME/my/example/package
 ```
 
 Execute all of a set of specified lint rules against all of the code in the