Description conflict of policy filter

[sagilio]

There may be two different structure descriptions of the policy filter:

casbin-website-v2/docs/PolicySubsetLoading.mdx

Lines 23 to 31 in 696e3db

	enforcer.InitWithAdapter("examples/rbac_with_domains_model.conf", adapter)
	filter := &fileadapter.Filter{
	P: []string{"", "domain1"},
	G: []string{"", "", "domain1"},
	}
	enforcer.LoadFilteredPolicy(filter)
	// The loaded policy now only contains the entries pertaining to "domain1".

casbin-website-v2/docs/Adapters.mdx

Lines 533 to 540 in 696e3db

	* `LoadFilteredPolicy` requires a `filter` as parameter. The filter should be something like this.
	```json
	{
	"p":[ [ "alice" ], [ "bob" ] ],
	"g":[ [ "", "book_group" ], [ "", "pen_group" ] ],
	"g2":[ [ "alice" ] ]
	}
	```

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[hsluoyz]

@sagilio why the 2nd code is 2D-array?