Make the rest of the services have a similar raw/user interface structure.

service/memcache/interface.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package memcache
 
 // Interface is the full interface to the memcache service.
+//
+// The *Multi methods may return a "github.com/luci/luci-go/common/errors".MultiError
+// if the rpc to the server was successful, but, e.g. some of the items were
+// missing. They may also return a regular error, if, for example, the rpc
+// failed outright.
 type Interface interface {
+        // NewItem creates a new, mutable, memcache item.
         NewItem(key string) Item
 
+        // Add puts a single item into memcache, but only if it didn't exist in
+        // memcache before.
         Add(item Item) error
+
+        // Set the item in memcache, whether or not it exists.
         Set(item Item) error
-        Get(key string) (Item, error)
+
+        // Get retrieves an item from memcache. Only the key field is read. The item
+        // will be mutated to contain the data in the memcache item.
+        Get(item Item) error
+
+        // Delete removes an item from memcache.
         Delete(key string) error
+
+        // CompareAndSwap accepts an item which is the result of Get() or GetMulti().
+        // The Get functions add a secret field to item ('CasID'), which is used as
+        // the "compare" value for the "CompareAndSwap". The actual "Value" field of
+        // the object set by the Get functions is the "swap" value.
+        //
+        // Example:
+        //   mc := memcache.Get(context)
+        //   itm := mc.NewItem("aKey")
+        //   mc.Get(itm) // check error
+        //   itm.SetValue(append(itm.Value(), []byte("more bytes")))
+        //   mc.CompareAndSwap(itm) // check error
         CompareAndSwap(item Item) error
 
         AddMulti(items []Item) error
         SetMulti(items []Item) error
-        GetMulti(keys []string) (map[string]Item, error)
+        GetMulti(items []Item) error
         DeleteMulti(keys []string) error
         CompareAndSwapMulti(items []Item) error
 
+        // Increment adds delta to the uint64 contained at key. If the memcache key
+        // is missing, it's populated with initialValue before applying delta (i.e.
+        // the final value would be initialValue+delta).
+        //
+        // Underflow caps at 0, overflow wraps back to 0.
+        //
+        // If key contains a value which is not exactly 8 bytes, it's assumed to
+        // contain non-number data and this method will return an error.
         Increment(key string, delta int64, initialValue uint64) (newValue uint64, err error)
+
+        // IncrementExisting is like Increment, except that the valu must exist
+        // already.
         IncrementExisting(key string, delta int64) (newValue uint64, err error)
 
+        // Flush dumps the entire memcache state.
         Flush() error
 
+        // Stats gets some best-effort statistics about the current state of memcache.
         Stats() (*Statistics, error)
+
+        Raw() RawInterface
 }