Fix #2060: `:manpage:` role uses incorrect style (node)

It should be use bold style to reference man pages in man format.
On other formats, it is still represented as emphasis.

tests/test_build_manpage.py

@@ -20,3 +20,4 @@ def test_all(app, status, warning):
 
     content = (app.outdir / 'SphinxTests.1').text()
     assert r'\fBprint \fP\fIi\fP\fB\en\fP' in content
+    assert r'\fBmanpage\en\fP' in content