mdoc(7) already uses the mandoc(1) -Ios argument in the footer line

when .Os has no argument, so do the same for man(7) when .TH has less
than four arguments; there is no reason to treat both differently.
Issue found following a question from Thomas Klausner <wiz at NetBSD>.

1 files changed, 3 insertions, 0 deletions

diff --git a/mandoc.1 b/mandoc.1
index 5b55b728..65487dba 100644
--- a/mandoc.1
+++ b/mandoc.1
@@ -85,6 +85,9 @@ Override the default operating system
 for the
 .Xr mdoc 7
 .Sq \&Os
+and for the
+.Xr man 7
+.Sq \&TH
 macro.
 .It Fl h
 Display only the SYNOPSIS lines.