Add documentation about the cmake plugin alerting the user that cmake doesn't set release mode by default.

[lengau]

What needs to get done

Add a note block to the cmake plugin reference calling out that it doesn't add any cmake parameters, so if the user wants to use release they should add -DCMAKE_BUILD_TYPE

Why it needs to get done

@dariuszd21 brought up the fact that the default may be debug.

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3455.

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[dariuszd21]

I duplicated it in #855 😅