{Graph} Fix #12797: ad app permission grant: Add prerequisite to the help message

[jiasli]

Description
Fix #12797: Cannot grant admin consent for an app role permission

If ad app permission grant is called for an app without a corresponding service principal, it will fail:

# Create the app
$ appId=$(az ad app create --display-name "myapp0630" --query appId -o tsv)

# Grant permission but it fails due to the lack of service principal 
$ az ad app permission grant --id $appId --api 00000003-0000-0000-c000-000000000000
Operation failed with status: 'Not Found'. Details: 404 Client Error: Not Found for url: https://graph.windows.net/54826b22-38d6-4fb2-bad9-b7b93a3e9c5a/oauth2PermissionGrants?$filter=clientId%20eq%20%27c9a8ddb5-d410-4de2-a7ca-0c5d55ff3a6b%27&api-version=1.6

To fix it, we need to create a corresponding service principal first:

# Create corresponding service principal
$ az ad sp create --id $appId

# Now granting permission works
$ az ad app permission grant --id $appId --api 00000003-0000-0000-c000-000000000000
{
  "clientId": "a201d009-477f-41d9-b683-799c29190706",
  "consentType": "AllPrincipals",
  "expiryTime": "2021-06-30T04:23:14.521719",
  "objectId": "CdABon9H2UG2g3mcKRkHBonI76O38TJFngGR4y0QOfQ",
  "odata.metadata": "https://graph.windows.net/54826b22-38d6-4fb2-bad9-b7b93a3e9c5a/$metadata#oauth2PermissionGrants/@Element",
  "odatatype": null,
  "principalId": null,
  "resourceId": "a3efc889-f1b7-4532-9e01-91e32d1039f4",
  "scope": "user_impersonation",
  "startTime": "2020-06-30T04:23:14.521719"
}

This PR adds this prerequisite to the help message.

[jiasli]

@yungezz, @yonzhan, Adding code owners @qianwens @arrownj.

[yonzhan]

LGTM

[yonzhan]

add to S172