GitLab

Problem reported by Eli Zaretskii in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00222.html
* doc/lispref/internals.texi (Garbage Collection):
Document vector sizes and slot counts more accurately.
* src/lisp.h: Omit header_size sanity check that was too picky.
Add some less-picky checks.