Call gdbpy_fix_doc_string_indentation for function help

If you invoke "help function _caller_is", you'll see that the help
text is indented strangely.  The fix for this is to add a call to
gdbpy_fix_doc_string_indentation in the appropriate spot, as is
already done for Python commands and parameters.

gdb/python/py-function.c

@@ -120,6 +120,8 @@ fnpy_init (PyObject *self, PyObject *args, PyObject *kwds)
 	      docstring = python_string_to_host_string (ds_obj.get ());
 	      if (docstring == NULL)
 		return -1;
+	      docstring
+		= gdbpy_fix_doc_string_indentation (std::move (docstring));
 	    }
 	}
     }