docs: fix the missing parameter description section in the smb.conf manpage

docs: fix the missing parameter description section in the smb.conf manpage

The smb.conf (5) manpage recently sometimes failed to contain the
contents of the description of each parameter section. The reason
was a unreliable chain of dependencies in the Makefile.

The error can be reproduced by touching manpages-3/smb.conf.5.xml
and then building the manpages.
Then smb.conf.5.xml is newer than any of the smbdotconf/*/*.xml
files and hence the intermediate inexistent parameters.*.xml
don't get generated.

This patch fixes this problem by introducing a phony "parameters"
target referencing the parameters.*.xml targets, so that they
get build unconditionally.

Fix bug #7997 (smb.conf.5 manpage truncated in 3.5.8).