Improve description for custom headers webhook authentication type

Problem/Motivation

Currently the module support all Dialogflow Fulfillment authentication types, including custom headers which however is not fully described what the key/value custom headers syntax is.

The module is expecting key -> value pairs delimited by colons, one custom header per line. For example:

In Dialogflow console:
Key: "Hello" , Value: "World"
Key: "Foo", Value: "Bar"

In Dialogflow webhook module configuation page:

Hello:World
Foo:Bar

No spaces, case sensitive.

Proposed resolution

Add description to the field explaining the syntax.