keyval: Improve some comments

 *

 * Semantics defined by reduction to JSON:

 *

 *   key-vals is a tree of objects and arrays rooted at object R

 *   where for each key-val = key-fragment . ... = val in key-vals

 *       R op key-fragment op ... = val'

 *       where (left-associative) op is

 *                 array subscript L[key-fragment] for numeric key-fragment

 *                 member reference L.key-fragment otherwise

 *             val' is val with ',,' replaced by ','

 *   and only R may be empty.

 *   key-vals specifies a JSON object, i.e. a tree whose root is an

 *   object, inner nodes other than the root are objects or arrays,

 *   and leaves are strings.

 *

 *   Duplicate keys are permitted; all but the last one are ignored.

 *   Each key-val = key-fragment '.' ... '=' val specifies a path from

 *   root to a leaf (left of '='), and the leaf's value (right of

 *   '=').

 *

 *   The equations must have a solution.  Counter-example: a.b=1,a=2

 *   doesn't have one, because R.a must be an object to satisfy a.b=1

 *   and a string to satisfy a=2.

 *   A path from the root is defined recursively:

 *       L '.' key-fragment is a child of the node denoted by path L

 *       key-fragment is a child of the tree root

 *   If key-fragment is numeric, the parent is an array and the child

 *   is its key-fragment-th member, counting from zero.

 *   Else, the parent is an object, and the child is its member named

 *   key-fragment.

 *

 * Key-fragments must be valid QAPI names or consist only of digits.

 *   This constrains inner nodes to be either array or object.  The

 *   constraints must be satisfiable.  Counter-example: a.b=1,a=2 is

 *   not, because root.a must be an object to satisfy a.b=1 and a

 *   string to satisfy a=2.

 *

 *   Array subscripts can occur in any order, but the set of

 *   subscripts must not have gaps.  For instance, a.1=v is not okay,

 *   because root.a[0] is missing.

 *

 *   If multiple key-val denote the same leaf, the last one determines

 *   the value.

 *

 * Key-fragments must be valid QAPI names or consist only of decimal

 * digits.

 *

 * The length of any key-fragment must be between 1 and 127.

 *

/*

 * Convert @key to a list index.

 * Convert all leading digits to a (non-negative) number, capped at

 * INT_MAX.

 * Convert all leading decimal digits to a (non-negative) number,

 * capped at INT_MAX.

 * If @end is non-null, assign a pointer to the first character after

 * the number to *@end.

 * Else, fail if any characters follow.

    }

    /*

     * Make a list from @elt[], reporting any missing elements.

     * Make a list from @elt[], reporting the first missing element,

     * if any.

     * If we dropped an index >= nelt in the previous loop, this loop

     * will run into the sentinel and report index @nelt missing.

     */