Remove "implementing ECS" reference from field descriptions

[ebeahan]

Several fields include this sentence in the description:

See the documentation section "Implementing ECS".

This section doesn't exist in the ECS docs, so I propose we remove this bit from the descriptions.

Fields:

• network.application
• network.protocol
• network.transport
• network.type

Relates partially to #745. The potential implementation of a lowercase normalization step/setting (also touched on in #745 (comment)) will be handled elsewhere.

[ebeahan]

Addressed in #1645