variable defined inside cache tags are not available outside

[dadaxr]

Add a warning in doc to mention that any defined variable inside {{ cache }} tags wont be available outside of them when the content is fetched from the cache...

It could be confusing from a DX perspective, as the first time the tag is executed (ie no cache), variable will be available outside, but then, it wont...

as an example, dont do this :

{{ myEntries = [] }}
{{ cache for="24 hours" }}
 {{ myEntries = { collection:xxx } {{# whatever #}}
{{ /cache }}

{{# here my entries will only be set at the first rendering but then it will be an empty array #}}
{{ myEntries }}
 ...
{{ /myEntries }}