Minor simplifications and doc for records

Minor simplifications and doc for records

* doc/lispref/records.texi (Records): Mention size limit.
* etc/NEWS: Mention records.
* src/alloc.c (allocate_pseudovector, allocate_record):
Prefer 'PSEUDOVECTOR_SIZE_MASK' to its definiens.
(allocate_record): Check arg range here, not in callers, as this
simplifies the code.  Use allocate_vectorlike instead of
allocate_vector, to avoid duplicate runtime tests.
(Fmake_record, record): Don't mention PSEUDOVECTOR_SIZE_BITS in
the doc string, as it is not visible to the user.
(Fmake_record, record, Fcopy_record):
Prefer make_lisp_ptr to XSETVECTOR.
(record): Broaden memcpy to copy the type, too.