gendoc: fix synopsis

The synopsis is used as an inline literal when generating the manpage.
There should not be any whitespace on the inside of the quotation
marks in inline literals.

Commands with an empty synopsis (such as tags) produces ``tags `` as
synopsis, which triggers a warning.

doc/gendoc.py

@@ -50,7 +50,7 b' def get_cmd(cmd):'
             s += ' ' + attr[2]
         else:
             s = attr[2]
-    d['synopsis'] = s
+    d['synopsis'] = s.strip()
 
     return d