Hard-code systemd unit Documentation= fields to the Streams mainline,
not a pinned release.
Per the user: don't want to update service files on every release (at
least not until OS packages handle that). systemd's Documentation= field
requires a real URI (no relative-path support), so unlike doc-to-doc
cross-references, this can't be solved with a relative link -- it's
hard-coded to /main/ instead, deliberately not tracking a release.
Fixes 10 files: the 6 real deployed templates under
Server/Unix/p4/common/etc/systemd/system/*.service.t (and one Unsupported
sample), which a broader search for "Documentation=" had missed the first
time (wrong file extension, .t not .service); plus the 3 illustrative
Server/Unix/setup/systemd/*.service sample files and its README.md, all
of which were stale-pinned to r25.1 (one, p4dtg_N.service.t, had already
been bumped to r26.1 -- inconsistent with the others, now all consistent
at /main/).