Imcomplete index in Texinfo format

**Your Godot version:**

I am building the docs from source, commit ac92bfb47

**Issue description:**

The index in the the texinfo docs generated using `make texinfo` is imcomplete, browsing the index page in Emacs gives the following result:


Looks like these entries are all coming from the same .rst file. I have not figured out why this document is different from the others. Do you have any suggestions?

I am building the Texinfo format because I would like to browse the document in Emacs, but the imcomplete index makes the process inefficient. I am also glad to help fix this problem, but I currently do not know how.