As a Jenkins plugin developer, when I provide an implementation of the GitSCMExtension extension point with environment variables via the populateEnvironmentVariables method, I also need to be able to provide documentation.
Such documentation I would expect to contribute to the page under the "Pipeline Syntax" -> "Global Variable Reference" menu under the "env" pipeline step.

Actually Jenkins core lookup for a buildEnv.jelly or buildEnv.groovy resource in the classpath of the plugin that provides an entension of EnvironmentContributor or SCM.
Since the SCM implementation (GitSCM) is provided by the git-plugin, any buildEnv.groovy resource in the plugin where the implementation of GitSCMExtension is placed is not taken into account.