meson: use alias_target for doc update commands

meson: use alias_target for doc update commands

This undoes part of 4c890ad3cc7b3445683d7b52bc00e4a58bef5e94: the
implementations of update-dbus-docs and update-man-rules are moved back to
man/meson.build, and alias_target() is used to keep the visible target names
unchanged.

The rules for man pages are reworked so that it's possible to invoke the
targets even if xstlproc is not available. After all, xsltproc is only needed
for the final formatted output, and not other processing.