docs: better documentation for silent make rules * doc/automake.texi (Options): Detailed description of the automake option `silent-rules' moved from here ... (Silent Make): ... into this new chapter, expanded, improved, and subdivided into ... (Make verbosity, Tricks For Silencing Make, Automake silent-rules Option): ... these new sections. (@menu, @detailmenu): Update. * tests/silent-configsite.test: New test, checking that the user can control default mode of silent-rules from config.site, as is documented in the manual. * tests/Makefile.am (TESTS): Updated.