doc: add note on unsupported include directive for source-read event content modification

[QSchulz]

Subject: doc: add note on unsupported include directive for source-read event content modification

Purpose

• Files included with the include directive have their content available in the source-read event but modifications will not make it to the output, so let's document it.

Relates

• #10678

Note

Does this need to be sent to master and/or 4.x branch too?