doc(util): Document the TimezoneGTM class

doc/api/util.rst

@@ -29,3 +29,6 @@ Miscellaneous
 
 .. automodule:: falcon.util
     :members: deprecated, dt_to_http, http_date_to_dt, to_query_str
+
+.. autoclass:: falcon.util.TimezoneGMT
+    :members:

falcon/util/time.py

@@ -2,15 +2,43 @@ import datetime
 
 
 class TimezoneGMT(datetime.tzinfo):
-    """Used in cookie response formatting"""
+    """GMT timezone class implementing the :py:class:`datetime.tzinfo` interface."""
 
     GMT_ZERO = datetime.timedelta(hours=0)
 
     def utcoffset(self, dt):
+        """Get the offset from UTC.
+
+        Args:
+            dt(datetime.datetime): Ignored
+
+        Returns:
+            datetime.timedelta: GMT offset, which is equivalent to UTC and
+                so is aways 0.
+        """
+
         return self.GMT_ZERO
 
     def tzname(self, dt):
+        """Get the name of this timezone.
+
+        Args:
+            dt(datetime.datetime): Ignored
+
+        Returns:
+            str: "GMT"
+        """
+
         return "GMT"
 
     def dst(self, dt):
+        """Return the daylight saving time (DST) adjustment.
+
+        Args:
+            dt(datetime.datetime): Ignored
+
+        Returns:
+            datetime.timedelta: DST adjustment for GMT, which is always 0.
+        """
+
         return self.GMT_ZERO