gendoc: support defaults on customopts a bit better

Without this, a customopt will very likely render like this:

```
-foo   does foo (default: <hgext.myextension.MyOpt object at 0x7f31...>)
```

I copied this logic from how this is handled in mercurial/help.py.

Differential Revision: https://phab.mercurial-scm.org/D9988

doc/gendoc.py

@@ -31,6 +31,7 b' from mercurial import ('
     commands,
     encoding,
     extensions,
+    fancyopts,
     help,
     minirst,
     pycompat,
@@ -86,6 +87,8 b' def get_opts(opts):'
         if b'\n' in desc:
             # only remove line breaks and indentation
             desc = b' '.join(l.lstrip() for l in desc.split(b'\n'))
+        if isinstance(default, fancyopts.customopt):
+            default = default.getdefaultvalue()
         if default:
             default = stringutil.forcebytestr(default)
             desc += _(b" (default: %s)") % default