Follow-up to GH-648
The genpepindex script requires the headers to be in a particular order.
In pep-0539.txt the order was different, breaking the build for everyone.
* Typo: "not not" (thanks Florent Hivert)
* Added a terminology note that hopefully should clear up confusion for new-comers to the PEP, as there have also been recent PEPs/discussions surrounding TLS in the HTTPS sense
* Adds a brief summary of the motivation to the abstract, along with a link to the relevant bug
* Updated the new API specification to reflect additions/changes since the last draft
* Added a usage example to the specification
* Added an explanation of the need for Py_tss_NEEDS_INIT to the rationalization
* Other minor wording updates and typo fixes