Clarify what "checking on bit" means

* doc/gpgme.texi (gpgme_sigsum_t summary): Clarify what "you
can check one bit means"

It was unclear which bit to test for is. And tests
with students have shown that they would "test" for
this one bit by using if(sig.summary == GPGME_SIGSUM_VALID)
which would fail because valid sigs are also SIGSUM_GREEN.

So lets give an example for people not used to checking bits and
clarify through this example which bit is meant to be checked.