help: deprecate ui.slash in favor of slashpath template filter (issue5572)

> For some reason, I thought someone (Mads?) said we had basically given
> up on ui.slash, and some commands didn't support it.
(from https://bz.mercurial-scm.org/show_bug.cgi?id=5572#c1)

So the ui.slash option doesn't always work and is somewhat confusing. Let's
make it clearer we won't improve the situation.

mercurial/help/config.txt

@@ -2184,6 +2184,8 b' User interface controls.'
     (default: True)
 
 ``slash``
+    (Deprecated. Use ``slashpath`` template filter instead.)
+
     Display paths using a slash (``/``) as the path separator. This
     only makes a difference on systems where the default path
     separator is not the slash character (e.g. Windows uses the