[Synapse]update new role assignment cmdlet help doc (#21756)

* update new role assignment cmdlet help doc

* Update ChangeLog.md

Documentational changes do not need a change log.

---------

Co-authored-by: Yeming Liu <[email protected]>

Author: yanjungao718

src/Synapse/Synapse/help/New-AzSynapseRoleAssignment.md

@@ -79,6 +79,7 @@ The cmdlet may call below Microsoft Graph API according to input parameters:
 
 >[!Note]
 > To create a role assignment for a service principal, pass the object ID of the principal in the ObjectId parameter.
+> When you are a user with permission to manage Azure RBAC role assignment on the workspace but not a Synapse Administrator, please create Synapse Administrator role by **-RoleDefinitionId** and **-ObjectId** directly but not by **-RoleDefinitionName**, for Synapse Administrator, RoleDefinitionId is "6e4bf58a-b8e1-4cc3-bbf9-d73143322b78", please take below Example 5 as reference. The reason for this is , when you try to add a "Synapse Administrator" role, the cmdlet needs to get the role ID from the role name which requires workspace read permission, which the current user does not have.
 
 ## EXAMPLES
 
@@ -111,6 +112,14 @@ $ws | New-AzSynapseRoleAssignment -RoleDefinitionName ContosoRole -SignInName Co
 
 This command assigns ContosoRole to the user whose principal name is ContosoName through pipeline.
 
+### Example 5
+```powershell
+$ws = Get-AzSynapseWorkspace -Name ContosoWorkspace
+$ws | New-AzSynapseRoleAssignment -RoleDefinitionId "6e4bf58a-b8e1-4cc3-bbf9-d73143322b78" -ObjectId ContosoObjectId
+```
+
+This command assigns Synapse Administrator role to the user whose object Id is ContosoObjectId through pipeline.
+
 ## PARAMETERS
 
 ### -AsJob