Change "language definition" to "language grammar" in manual.

Change "language definition" to "language grammar" in manual.

"Language grammar" is apparently the more widely used term among
people talking about tree-sitter.  To avoid confusion, I think it's
good to use that rather than "language definition", which is a term
that I made up :-)

Feel free to revert this if it is too much change.

* doc/lispref/elisp.texi:
* doc/lispref/modes.texi:
* doc/lispref/parsing.texi:
* lisp/treesit.el: Change "language definition" to "language grammar".