config/alias.txt: document alias accepting non-command first word

One can see that an alias that begins with a non-command first word,
such as `loud-rebase = -c commit.verbose=true rebase`, is permitted.
However, this isn't immediately obvious to users as alias instances
typically begin with a command.

Document the fact that an alias can begin with a non-command first word
so that users will be able to discover that this is a feature.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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