doc: clarify <src> in refspec syntax

We explicitly avoid saying "ref <src>" when introducing the source side of a refspec, because it can be a fully-spelled hexadecimal object name, and it also can be a pattern that is not quite a "ref". But we are loose when we introduce <dst> and say "ref <dst>", even though it can also be a pattern. Let's omit "ref" also from the destination side. Clarify that <src> can be a ref, a (limited glob) pattern, or an object name. Even though the very original design of refspec expected that '*' was used only at the end (e.g., "refs/heads/*" was expected, but not "refs/heads/*-wip"), the code and its use evolved to handle a single '*' anywhere in the pattern. Update the text to remove the mention of "the same prefix". Anything that matches the pattern are named by such a (limited glob) pattern in <src>. Also put a bit more stress on the fact that we accept only one '*' in the pattern by saying "one and only one `*`". Helped-by: Monika Kairaitytė <monika@kibit.lt> Signed-off-by: Junio C Hamano <gitster@pobox.com>