alias: fixes exception when displaying translated help text
__doc__ of aliased command shouldn't cointain non-ASCII characters,
because it'll be gettext-ed later by commands.help_().
Here gettext can raise UnicodeDecodeError.
Once concatenated two translatable strings into one, it become untranslatable.
So this patch moves 'alias for:' from dispatch.cmdalias to commands.help_,
where help texts are translated.
'alias for:' was introduced by 6e8a16dd3e30.