GCC seems to be using "deprecated" consistently over "obsoleted", so
use -Wdeprecated rather than -Wobsolete.
* src/complain.h (warnings): Add Wdeprecated.
* src/complain.c (warnings_print_categories): Adjust.
* src/getargs.c: Likewise.
* doc/bison.texi: Document it.
* src/scan-code.l: Use this category for the trailing ';' support.
* tests/actions.at: Adjust expected output.