Documentation: clarify signature verification

Clarify that "merge --verify-signatures" checks the signature on the
tip commit of the history being merged.

Uniformise the vocabulary used wrt. key/signature validity with OpenPGP:
- a signature is valid if made by a key with a valid uid;
- in the default trust-model, a uid is valid if signed by a trusted key;
- a key is trusted if the (local) user set a trust level for it.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Keller Fuchs   <KellerFuchs@hashbang.sh>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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