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