doc: gendoc.py creates valid output for option descriptions with newlines

gendoc.py did not handle the hanging indentation for descriptions. Work around
this by joining all in one single line (same as in minirst since previous
patch).

This problem occurred when translations of option lines were very long. Do not
bother the translators with this detail.

On a long option description, the translator continued on a new line as usual.
gendoc.py created invalid rst syntax like this:

-o, --option
    Description line 1
description line 2

The new output is:

-o, --option
    Description line 1 description line 2

The lines could theoretically become very long, but line breaking is handled
when generating the final documentation.

doc/gendoc.py

@@ -50,6 +50,9 b' def get_opts(opts):'
             allopts[-1] += " <%s[+]>" % optlabel
         elif (default is not None) and not isinstance(default, bool):
             allopts[-1] += " <%s>" % optlabel
+        if '\n' in desc:
+            # only remove line breaks and indentation
+            desc = ' '.join(l.lstrip() for l in desc.split('\n'))
         desc += default and _(" (default: %s)") % default or ""
         yield (", ".join(allopts), desc)