Provide clear documentation on allowed characters and limitations for custom keys update for 2.15

Currently there is no documentation describing the limitations for any custom keys that can be created by API.

Examples: app_id, app_key, user_key.

In the admin portal UI there is a note when creating a custom key that states the following:

Only alphanumeric characters [0-9, a-z, A-Z], hyphen-minus ,
between 5 and 256 characters long. No spaces allowed.

This accounts for the app_key & client_secret but when creating a new key or even a new application plus keys on the 3scale API there needs to be clearer documentation stating what is/isn't allowed.

Important: This also needs to be added to the ActiveDocs.

Note: Documentation was improved for 2.14 and 2.13, we need to extend definition based on changes for 2.15