[infra] Add more documentation on builder names in tools/bots/test_matrix.json

Change-Id: I975ee88145ec59a4579d458f144b7de19fe5e10b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/415880
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Alexander Thomas <[email protected]>

Author: osa1

tools/bots/README.md

@@ -54,10 +54,10 @@ an `options` field.
     "options": {
       "compiler": "dartk",
       "mode": "release",
-}},
+  }
+},
 ```
 
-
 ### Builder Configurations
 
 The builder configurations describes all test configurations a specific builder
@@ -131,6 +131,19 @@ option. Options can be runtimes (e.g. "chrome"), architectures (e.g. x64) and
 operating system families (e.g. win). For each valid option, additional
 arguments are passed to the `tools/build.py` script.
 
+Parsing of builder names is done in [1]. A name like
+"front-end-nnbd-win-release-x64" is parsed into variables:
+
+- `info`: `"front-end-nnbd"`
+- `os`: `"win"`
+- `mode`: `"release"`
+- `arch`: `"x64"`
+
+Valid values of these variables are defined in [2].
+
+[1]: https://dart.googlesource.com/recipes/+/535cf6e8ef1e11449acfc13c7bc61a530b765bd8/recipe_modules/dart/api.py#886
+[2]: https://dart.googlesource.com/recipes/+/535cf6e8ef1e11449acfc13c7bc61a530b765bd8/recipe_modules/dart/__init__.py
+
 ### Adding a new builder
 To add a builder: