Add some documentation, simplify the all target, and add a clean target.
Add some documentation, simplify the all target, and add a clean target.

LuaDist/lrexlib-posix,LuaDist/lrexlib-posix,LuaDist/lrexlib-tre,LuaDist/lrexlib-gnu,LuaDist/lrexlib-gnu,LuaDist/lrexlib-tre,LuaDist/lrexlib-oniguruma,StoneDot/lrexlib,LuaDist/lrexlib-pcre,LuaDist/lrexlib-pcre,LuaDist/lrexlib-oniguruma,StoneDot/lrexlib