document pthread structure ABI constraints in comments

document pthread structure ABI constraints in comments

in the original submission of the patch that became commit
7c709f2d4f9872d1b445f760b0e68da89e256b9e, and in subsequent reading of
it by others, it was not clear that the new member had to be inserted
before canary_at_end, or that inserting it at that location was safe.
add comments to document.