Document which toolchains must be listed in `~/.m2/toolchains.xml` to build locally

On a Mac when I run `mvn clean verify` on the latest master branch I get an error:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-toolchains-plugin:3.2.0:toolchain (default) on project error_prone_parent: Cannot find matching toolchain definitions for the following toolchain types:
[ERROR] jdk [ version='25' ]
[ERROR] Please make sure you define the required toolchains in your ~/.m2/toolchains.xml file.
```

I tried changing the value in `pom.xml` to 23 or 21 (both of which I have installed) but I got a similar error. I also ran `mvn toolchains:generate-jdk-toolchains-xml -Dtoolchain.file=~/.m2/toolchains.xml` to try to generate the `toolchains.xml` file but still got the same error.

Are there docs somewhere on steps required to build locally and properly specify the toolchains? Thanks!