docs: Add policy variables for resource and conditions (#10278)

Bonus fix adds LDAP policy variable and clarifies the
usage of policy variables for temporary credentials.

fixes #10197

pkg/bucket/policy/condition/ldap.go

@@ -0,0 +1,22 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package condition
+
+const (
+	// LDAPUser - LDAP username, in MinIO this value is equal to your authenticating LDAP user.
+	LDAPUser Key = "ldap:user"
+)