fix pool never closing when evictor is enabled and pool.close not called

clear up some documentation and comments

lib/Pool.js

@@ -469,17 +469,21 @@ class Pool extends EventEmitter {
 
   /**
    * Disallow any new acquire calls and let the request backlog dissapate.
-   * Resolves once all resources are returned to pool and available...
-   * should probably be called "drain work"
+   * The Pool will no longer attempt to maintain a "min" number of resources
+   * and will only make new resources on demand.
+   * Resolves once all resource requests are fulfilled and all resources are returned to pool and available...
+   * Should probably be called "drain work"
    * @returns {Promise}
    */
   drain () {
-    // disable the ability to put more work on the queue.
     this._draining = true
     return this.__allResourceRequestsSettled()
       .then(() => {
         return this.__allResourcesReturned()
       })
+      .then(() => {
+        this._descheduleEvictorRun()
+      })
   }
 
   __allResourceRequestsSettled () {
@@ -511,13 +515,10 @@ class Pool extends EventEmitter {
    *
    */
   clear () {
-    this._descheduleEvictorRun()
-    // wait for outstanding factory.create to complete
-    // FIXME: any new factory.create calls created after this will not
-    // handled by us
     const reflectedCreatePromises = Array.from(this._factoryCreateOperations)
     .map(reflector)
 
+    // wait for outstanding factory.create to complete
     return this._Promise.all(reflectedCreatePromises)
     .then(() => {
       // Destroy existing resources