[doc] make examples for XeTeX notation uppercase
perform match on other font names if family but not subfamily matches

preliminary fix for issue #26

here’s an example that works now but either broke (pre-v1.3) or
retrieved the wrong shape with ``/B``:

    \ifdefined\directlua\input luaotfload.sty\fi
    %% this should be mono bold
    \font\libertinemono="Linux Libertine Mono O" at 42pt
    foo {\libertinemono bar} baz\endgraf
    %% this should be bold, but isn’t
    \font\myriadbold="Myriad Pro/B" at 42pt
    foo {\myriadbold bar} baz\endgraf
    %% this is bold
    \font\minionbold="Minion Pro/B" at 42pt
    foo {\minionbold bar} baz\endgraf
    \bye

also, I refactored parts of the matching function for more clarity