Improve clarity of USE_LSB_TAG definition.

Problem reported by Lee Duhem.  Suggestion by Andreas Schwab in:
http://lists.gnu.org/archive/html/emacs-devel/2014-11/msg02222.html
* lisp.h (USE_LSB_TAG): Define in terms of the (simpler)
VAL_MAX / 2 rather than in terms of the (more complicated)
EMACS_INT_MAX >> GCTYPEBITS, and adjust commentary to match.
Spelling fix.