Order of execution for preconditions of node is unspecified

[nyibbang]

In the documentation about pre conditions, there is no specification of order of execution between the preconditions of a node:

btcpp_website/docs/guides/pre_post_conditions.md

Lines 26 to 33 in a63ca21

	## Pre conditions
	| Name | Description |
	|-------------|---------|
	| **_skipIf** | Skip the execution of this Node, if the condition is true |
	| **_failureIf** | Skip and return FAILURE, if the condition is true |
	| **_successIf** | Skip and return SUCCESS, if the condition is true |
	| **_while** | Same as _skipIf, but may also interrupt a RUNNING Node if the condition becomes false. |

Preconditions can easily overlap in real trees (meaning for example, both a _failureIf and a _successIf can become true at the same time) and it can become difficult to know what happens then.

After checking the implementation of the library :

https://github.com/BehaviorTree/BehaviorTree.CPP/blob/cdaca9c2c68f4b3aa187040f72b02b8dc8a1ed25/include/behaviortree_cpp/tree_node.h#L46-L54

enum class PreCond
{
  // order of the enums also tell us the execution order
  FAILURE_IF = 0,
  SUCCESS_IF,
  SKIP_IF,
  WHILE_TRUE,
  COUNT_
};

It seems the order of execution is specified here. I have tested on v4.8.2 and I observed the same results. I believe adding the order in the documentation would help users of the library.