Merge pull request #418 from piotrgo/azurerm_pgsql_docu2

Documentation update for Azure DB for PostgreSQL

website/docs/r/postgresql_database.html.markdown

@@ -58,7 +58,7 @@ The following arguments are supported:
 
 * `charset` - (Required) Specifies the Charset for the PostgreSQL Database, which needs [to be a valid PostgreSQL Charset](https://www.postgresql.org/docs/current/static/multibyte.html). Changing this forces a new resource to be created.
 
-* `collation` - (Required) Specifies the Collation for the PostgreSQL Database, which needs [to be a valid PostgreSQL Collation](https://www.postgresql.org/docs/current/static/collation.html). Changing this forces a new resource to be created.
+* `collation` - (Required) Specifies the Collation for the PostgreSQL Database, which needs [to be a valid PostgreSQL Collation](https://www.postgresql.org/docs/current/static/collation.html). Note that Microsoft uses different [notation](https://msdn.microsoft.com/library/windows/desktop/dd373814.aspx) - en-US instead of en_US. Changing this forces a new resource to be created.
 
 ## Attributes Reference