Fixup images and add notes in docs (#54)

* Update SequenceWithMemory.svg

* Fix FetchBeer

* Temporary solutions for incomplete docs.

* fixup! Fix typo in SequenceNode.md (#53)

Author: Eric-SHENNONGSHI

corrected_behavior_tree.svg

@@ -5,6 +5,8 @@ A decorator is a node that must have a single child.
 It is up to the Decorator to decide if, when and how many times the child should be
 ticked.
 
+> Some nodes are not listed yet. See [decorators](https://github.com/BehaviorTree/BehaviorTree.CPP/tree/master/include/behaviortree_cpp/decorators) on Github for complete definitions.
+
 ## Inverter
 
 Tick the child once and return SUCCESS if the child failed or FAILURE if

docs/learn-the-basics/images/FetchBeer.svg

@@ -1,4 +1,4 @@
-# Fallbacks
+# Fallbacks (Controls)
 
 This family of nodes are known as "Selector" or "Priority"
 in other frameworks.
@@ -36,6 +36,8 @@ To understand how the two ControlNodes differ, refer to the following table:
   same child is ticked again. Previous siblings, which returned FAILURE already,
   are not ticked again.
 
+> Some nodes are not listed yet. See [controls](https://github.com/BehaviorTree/BehaviorTree.CPP/tree/master/include/behaviortree_cpp/controls) on Github for complete definitions.
+
 ## Fallback
 
 In this example, we try different strategies to open the door. 

docs/nodes-library/DecoratorNode.md

@@ -34,6 +34,8 @@ To understand how the three ControlNodes differ, refer to the following table:
   same child is ticked again. Previous siblings, which returned SUCCESS already,
   are not ticked again.
 
+> Some nodes are not listed yet. See [controls](https://github.com/BehaviorTree/BehaviorTree.CPP/tree/master/include/behaviortree_cpp/controls) on Github for complete definitions.
+
 ## Sequence
 
 This tree represents the behavior of a sniper in a computer game.
@@ -45,7 +47,7 @@ This tree represents the behavior of a sniper in a computer game.
 
 This node is particularly useful to continuously check Conditions; but 
 the user should also be careful when using asynchronous children, to be
-sure that they are not ticked more often that expected.
+sure that they are not ticked more often than expected.
 
 Let's take a look at another example:
 
@@ -72,4 +74,4 @@ If the action __GoTo(B)__ fails, __GoTo(A)__ will not be ticked again.
 On the other hand, __isBatteryOK__ must be checked at every tick, 
 for this reason its parent must be a `ReactiveSequence`.
 
-![SequenceWithMemory](images/SequenceStar.svg)
+![SequenceWithMemory](images/SequenceWithMemory.svg)

docs/nodes-library/FallbackNode.md

@@ -45,7 +45,7 @@ This tree represents the behavior of a sniper in a computer game.
 
 This node is particularly useful to continuously check Conditions; but 
 the user should also be careful when using asynchronous children, to be
-sure that they are not ticked more often that expected.
+sure that they are not ticked more often than expected.
 
 Let's take a look at another example: