docs: add build capture group to regex docs (#12065)

lib/versioning/regex/readme.md

@@ -1,9 +1,11 @@
-Regular Expression Versioning is designed to be like a flexible fallback versioning approach is Renovate's other versioning schemes don't do the job.
+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 valid capture groups for `regex` versioning are:
+The `regex` scheme makes use of Regular Express 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 updated, 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.
-- `prerelease`: this value, if captured, will mark a given release as a prerelease (eg. unstable). If this value is captured and you have configured `"ignoreUnstable": true`, the given release will be skipped.
+- `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.
+- `build`: this capture group can be used after you've already used the `major`, `minor` and `patch` capture groups and need a fourth version component. `build` updates are handled like `patch` updates.
+- `prerelease`: this value, if captured, will mark a given release as a prerelease (e.g. unstable). If this value is captured and you have configured `"ignoreUnstable": true`, the given release will be skipped.
 - `compatibility`: this value defines the "build compatibility" of a given dependency. A proposed Renovate update will never change the specified compatibility value. For example, if you are pinning to `1.2.3-linux` (and `linux` is captured as the compatibility value), Renovate will not update you to `1.2.4-osx`.
 
 The compatibility concept was originally introduced for Docker versioning but sometimes package authors may use/misuse suffixes to mean compatibility in other versioning schemes.