[OPENJDK-1964] Clarify permitted format for MAVEN_REPOS

Expand the documentation for MAVEN_REPOS to make it clear that
it must be specified in uppercase, and that hyphens/dashes are
replaced by underscores.

Signed-off-by: Jonathan Dowland <[email protected]>

Author: jmtd

modules/maven/default/module.yaml

@@ -62,8 +62,15 @@ envs:
     MAVEN_REPO_URL. The default is to generate a random ID.
     See MAVEN_REPOS for specifying multiple repositories.
 - name: "MAVEN_REPOS"
-  example: "dev-one,qe-two"
-  description: "If set, multi-repo support is enabled, and other MAVEN_REPO_* variables will be prefixed. For example: DEV_ONE_MAVEN_REPO_URL and QE_TWO_MAVEN_REPO_URL"
+  example: "DEV-ONE,QE-TWO"
+  description: >
+    Enables multi-repo support. Specify a comma-delimited list of capitalized
+    repository identifiers. The configuration for each repository will be
+    determined by correspondingly prefixed `MAVEN_REPO_*` variables. Any dashes
+    in repository names will be replaced with underscores. For example:
+    Specifying `DEV-ONE,QE-TWO` configures two repositories and their
+    URLs will be read from `DEV_ONE_MAVEN_REPO_URL` and
+    `QE_TWO_MAVEN_REPO_URL`.
 - name: "prefix_MAVEN_REPO_ID"
   example: "my-repo-id"
   description: "Maven repository id"