Update AST docs for :inbounds and :boundscheck.

[ci skip]

doc/devdocs/ast.rst

@@ -139,12 +139,17 @@ These symbols appear in the ``head`` field of ``Expr``\s in lowered form.
 ``leave``
  pop exception handlers. ``args[1]`` is the number of handlers to pop.
 
-``boundscheck``
+``inbounds``
  controls turning bounds checks on or off. A stack is maintained; if the
  first argument of this expression is true or false (``true`` means bounds
  checks are enabled), it is pushed onto the stack. If the first argument is
  ``:pop``, the stack is popped.
 
+``boundscheck``
+ indicates the beginning or end of a section of code that performs a bounds
+ check. Like ``inbounds``, a stack is maintained, and the second argument
+ can be one of: ``true``, ``false``, or ``:pop``.
+
 ``copyast``
  part of the implementation of quasi-quote. The argument is a surface syntax
  AST that is simply copied recursively and returned at run time.