docs: add missing wasm parameters

[kayx23]

Wasm's priority, file, and http_request_phase were previously documented but got removed in #415 which switched to using helm-docs to generate the docs.

The missed configuration could lead to:

This PR adds these parameters back.

[kayx23]

These parameters are indeed missing from the docs but the updated parameter levels seem wrong.

https://github.com/apache/apisix-helm-chart/blob/master/charts/apisix/templates/configmap.yaml#L297-L301

[nic-6443]

priority, file, and http_request_phase are per-plugin properties in APISIX's config, not top-level wasm settings. The existing configmap template just does toYaml .Values.apisix.wasm.plugins, so these top-level values won't be used in the generated config at all.

The correct structure should have these as fields within each plugin entry:

wasm:
  enabled: false
  plugins:
    - name: "example-plugin"
      priority: 7999
      file: "/path/to/plugin.wasm"
      http_request_phase: "rewrite"

As-is, setting apisix.wasm.priority or apisix.wasm.file would have no effect on the rendered config. The docs should show the correct per-plugin structure instead.