Fix overenthusiastic header size check

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.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/lispref/internals.texi b/doc/lispref/internals.texi
index 3fe28446eaf..d42e2444e68 100644
--- a/doc/lispref/internals.texi
+++ b/doc/lispref/internals.texi
@@ -382,7 +382,7 @@ This is used for convenience and equals to @code{sizeof (char)}.
 The total size of all string data in bytes.
 
 @item vector-size
-Internal size of a vector header, i.e., @code{sizeof (struct Lisp_Vector)}.
+Size in bytes of a vector of length 1, including its header.
 
 @item used-vectors
 The number of vector headers allocated from the vector blocks.
@@ -392,6 +392,8 @@ Internal size of a vector slot, always equal to @code{sizeof (Lisp_Object)}.
 
 @item used-slots
 The number of slots in all used vectors.
+Slot counts might include some or all overhead from vector headers,
+depending on the platform.
 
 @item free-slots
 The number of free slots in all vector blocks.