[DOC] Add reusing context generated in expression to doc #1372
Description
Include documentation about how to reuse context generated by expressions on subsequent expressions.
Preferred place:
https://github.com/telefonicaid/iotagent-node-lib/blob/master/doc/api.md#expression-language-support
Evaluate to include a new section talking about context
Add a note about expressions execution orders (by array order) and warning about Interdependencies. Context is evaluated in order
Another point, that is not included in the documentation, in that multientity plugin is now supporting expressions to generate the entity name destination
Related PR: #1370