doc: interpret-trailers: explain key format

A trailer key must consist of ASCII alphanumeric characters and
hyphens *only*. Let’s document it explicitly instead of relying on
readers being conservative and painting their trailers by numbers
(by the documentation examples).

The previous commit for “key–value pairs” allows us to segue right into
describing these lines as consisting of a key and a value, which is our
opening to describing the key format.

Just like *trailer* we emphasize these two first standalone word
mentions. They are then mostly used in placeholders throughout the rest
of the document (<key> and <value>).

Reported-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-interpret-trailers.adoc

@@ -27,7 +27,10 @@ Signed-off-by: Alice <alice@example.com>
 Signed-off-by: Bob <bob@example.com>
 ------------------------------------------------
 
-the last two lines starting with `Signed-off-by` are trailers.
+the last two lines starting with `Signed-off-by` are trailers. These two
+trailers have the _key_ `Signed-off-by` and a _value_ (Alice and Bob).
+The key must consist of only ASCII alphanumeric characters and hyphens
+(`-`). The hyphens serve as interword separators.
 
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