docs(manager/flux): improve readme (#17507)

lib/modules/manager/flux/readme.md

-This manager parses [Flux](https://fluxcd.io/) YAML manifests and:
+This manager parses [Flux](https://fluxcd.io/) YAML manifests and supports:
 
-1. Extracts `helm` dependencies from `HelmRelease` resources
-2. Extracts `github-releases` dependencies from system manifests (`flux-system/gotk-components.yaml` files) and regenerates them when new versions of Flux are available
+1. [`HelmRelease`](https://fluxcd.io/docs/components/helm/helmreleases/) resources
+1. Flux [system](https://fluxcd.io/docs/installation) manifests
+
+### HelmRelease support
+
+Extracts `helm` dependencies from `HelmRelease` resources.
 
 The `flux` manager will only extract `helm` dependencies for `HelmRelease` resources linked to `HelmRepository` sources.
 The following configurations are currently unsupported:
@@ -16,12 +20,19 @@ In addition, for the `flux` manager to properly link `HelmRelease` and `HelmRepo
 
 Namespaces will not be inferred from the context (e.g. from the parent `Kustomization`).
 
+### Flux system manifests support
+
+Support updating Flux system manifests generated during [Flux installation](https://fluxcd.io/docs/installation/#customize-flux-manifests).
+
 Updating system manifests requires that either:
 
 1. The `flux` tool is pre-installed, or
-2. You run a Docker image based on [containerbase](https://github.com/containerbase), such as the official Renovate images, and have `binarySource=install` configured
+1. You run a Docker image based on [containerbase](https://github.com/containerbase), such as the official Renovate images, and have `binarySource=install` configured
+
+### Non-configured fileMatch
 
 By default, the `flux` manager will only match `flux-system/gotk-components.yaml` (i.e. system manifest) files.
+
 This is because there is no commonly accepted file/directory naming convention for Flux manifests and we don't want to check every single `*.yaml` file in repositories just in case some of them have Flux definitions.
 
 If most `.yaml` files in your repository are Flux manifests, then you could add this to your config:
@@ -44,4 +55,6 @@ If instead you have all your Flux manifests inside a `flux/` directory, you woul
 }
 ```
 
+### Versioning
+
 If you need to change the versioning format, read the [versioning](https://docs.renovatebot.com/modules/versioning/) documentation to learn more.