Document custom domain parameter naming in RBAC with domains

[Copilot]

PR apache/casbin#1611 fixed a bug where renaming the domain parameter from dom to arbitrary names (e.g., tenant, organization) caused enforcement failures. This adds documentation for that fix.

Changes

• docs/RBACWithDomains.mdx: Added "Custom Domain Parameter Names" section explaining that any parameter name works seamlessly, with SetFieldIndex() only required for non-standard positions

• docs/SyntaxForModels.mdx: Added "Domain Parameters in Matchers" subsection showing automatic domain token detection

Example

Users can now freely use custom domain names:

[request_definition]
r = sub, tenant, obj, act

[policy_definition]
p = sub, tenant, obj, act

[role_definition]
g = _, _, _

[matchers]
m = g(r.sub, p.sub, r.tenant) && keyMatch(r.tenant, p.tenant) && r.obj == p.obj && r.act == p.act

Casbin automatically detects the domain parameter regardless of name. SetFieldIndex() is only needed when placing the domain parameter in a non-standard position (not second).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[docs] add docs for the PR: 1611</issue_title>
<issue_description>Improve the related docs page based on the newly merged PR: apache/casbin#1611
You should write high-quality human-like docs for a modern OSS software. Don't write in the GPT-flavor. Don't write too long, keep it tidy docs. Don't use itemized list to describe things like "Use Cases", "Best practice" or "what it can do"
don't change yarn.lock
Avoid adding new docs pages, try the best to add content to the suitable existing docs pages
Don't add too long text, keep it short and to the point
Don't add the docs in a template like one section name and one text paragraph. You need to organize the content to be user-friendly and not boring for users to read
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [docs] add docs for the PR: 1611 #432

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