rtutil: Add docstring with basic rationale.

conservancy_beancount/rtutil.py

@@ -30,6 +30,17 @@ from typing import (
 RTId = Union[int, str]
 
 class RT:
+    """RT utility wrapper class
+
+    Given an RT client object, this class provides common functionality for
+    working with RT links in Beancount metadata:
+
+    * Parse links
+    * Verify that they refer to extant objects in RT
+    * Convert metadata links to RT web links
+    * Cache results, to reduce network requests
+    """
+
     PARSE_REGEXPS = [
         re.compile(r'^rt:([0-9]+)(?:/([0-9]+))?/?$'),
         re.compile(r'^rt://ticket/([0-9]+)(?:/attachments?/([0-9]+))?/?$'),