bug#34073: 26.1; Missing documentation for minor mode hook behavior

[Piotr Kaznowski]

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I couldn't find essential information about minor mode hooks behavior, namely 
that "The minor mode's hook is called both when activating and deactivating the 
minor mode" (as stated here: https://emacs.stackexchange.com/a/47092/17425).

It isn't mentioned in the manual (sections "Minor Modes" and "Hooks"), nor in 
the docstring for `add-hook' function.

Please consider adding this information in the relevant sections of the docs.
Thank you.