Sync VSC documentation with reality in varnish.m4

This was documented following the naming convention in tree, but
implemented differently. In order not to change the behavior, only
the documentation is updated.

Closes #3232

varnish.m4

@@ -437,8 +437,8 @@ clean-vsc-$1:
 #     @BUILD_VSC_FOO@
 #     @BUILD_VSC_BAR@
 #
-# They take care of turning VSC_foo.vsc and VCS_bar.vsc into C code and
-# RST documentation.
+# They take care of turning foo.vsc and bar.vsc into C code and RST
+# documentation.
 #
 # Just like the vcc_*_if.[ch] files, you need to manually add the generated
 # sources to the appropriate VMODs:
@@ -454,6 +454,11 @@ clean-vsc-$1:
 #
 #     .. include:: VSC_foo.rst
 #
+# Doing so, you can add the generated RST as a dependency of the manual in
+# Makefile.am:
+#
+#     vmod_baz.3: VSC_foo.rst
+#
 # That should be all you need to do to start implementing custom counters.
 #
 AC_DEFUN([VARNISH_COUNTERS], [