Update incoming email docs to clarify key use

What does this MR do?

Service Desk was identified as difficult for non-engineers to set up and test locally in #322089 (closed).

The developer guidance around setting up incoming email, while accurate, doesn't specify that the +%{key} placeholder is required for some features to work properly, including Service Desk. It would be better to simply state that, while incoming email will work without the placeholder, some features will not.

It's not clear if there's any benefit to omitting the placeholder anyway.

Related issues

#322089 (closed)

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