Previous code expected only one XML per module, which is not the case for
e.g. mono or gdnative.
Also add newline after signal description to fix rst warning, and make the
script Python 3-compatible.
[ci skip]
Fixes a number of issues:
- Headings underliners now have the correct length
- Newline+Tabs in descriptions are replaced by two newlines to make a proper paragraph
- [br] are replaced by two newlines, making a proper paragraph
- Properly parse internal hyperlinks in constants description
- Fix broken internal links due to missing newlines
- Show method header even when it has no description, to have something to reference in hyperlinks