docs(versioning:regex): mention numeric capture group requirement

lib/modules/versioning/regex/readme.md

@@ -1,6 +1,7 @@
 Regular Expression Versioning is designed to be a flexible fallback versioning approach if Renovate's other versioning schemes don't do the job.
 
-The `regex` scheme makes use of Regular Express capture groups.
+The `regex` scheme makes use of Regular Expression capture groups.
+
 The valid capture groups for `regex` versioning are:
 
 - `major`, `minor`, and `patch`: at least one of these must be provided. When determining whether a package has updates, these values will be compared in the standard semantic versioning fashion. If any of these fields are omitted, they will be treated as if they were `0` -- in this way, you can describe versioning schemes with up to three incrementing values.
@@ -11,6 +12,9 @@ The valid capture groups for `regex` versioning are:
 
 The compatibility concept was originally introduced for Docker versioning but sometimes package authors may use/misuse suffixes to mean compatibility in other versioning schemes.
 
+**Important: all capture groups must contain only purely numeric values.**
+Even if there is a string prefix which is identical in all available versions, it must not be part of the capture group. For example a `build` capture group containing `r4` cannot be evaluated as number; Renovate cannot compare the `build` in this case. The capture group must be `4` instead.
+
 Here is an example of using `regex` versioning to correct behavior of the `guava` Maven package, which misuses suffixes as compatibility indicators:
 
 ```json