[DOC] Add to special measure name metadata key

[mapedraza]

iotagent-node-lib/doc/api.md

Lines 169 to 177 in c57d892

	## Special measures and attributes names
	In case of arriving measures with name `id` or `type`, they are automatically transformed to `measure_id` and
	`measure_type` attributes at Context Broker update. The reason behind this is to avoid to collide with the original
	entity ID and type, as mechanism that enable store measure values from parameters called with the same name. It only
	applies to autoprovisioned attributes and is also available at JEXL context with the same name (`measure_id` or
	`measure_type`).
	In case of provisioning attributes using `id` or `type` as names (please don't do that ;), they are ignored.

EDIT:

Add also metadata here:

iotagent-node-lib/doc/api.md

Lines 593 to 602 in c57d892

	The agent provides additional information, hereafter referred as **context**, in order to be used to evaluate the
	expression. In all cases the following data is available to all expressions:
	- `id`: device ID
	- `entity_name`: NGSI entity Name (principal)
	- `type`: NGSI entity type (principal)
	- `service`: device service
	- `subservice`: device subservice
	- `staticAttributes`: static attributes defined in the device or config group

[AlvaroVega]

done in #1641. Should we close this?