Improve test descriptions

spec/lib/gitlab/exclusive_lease_spec.rb

 require 'spec_helper'
 describe Gitlab::ExclusiveLease do
-  it 'is exclusive' do
+  it 'cannot obtain twice before the lease has expired' do
     lease = Gitlab::ExclusiveLease.new(unique_key, timeout: 3600)
     expect(lease.try_obtain).to eq(true)
     expect(lease.try_obtain).to eq(false)
   end
-  it 'expires' do
+  it 'can obtain after the lease has expired' do
     timeout = 1
     lease = Gitlab::ExclusiveLease.new(unique_key, timeout: timeout)
-    lease.try_obtain
-    sleep(2 * timeout)
+    lease.try_obtain # start the lease
+    sleep(2 * timeout) # lease should have expired now
     expect(lease.try_obtain).to eq(true)
   end