bug#15547: 24.3.50; doc string of `isearch-cmds'

[Drew Adams]

> 1. The first element of the vector is missing from the description.  It
> is the symbol `cl-struct-isearch--state'.  The vector has 13 elements,
> not 12.
> 
> 2. It would be a little clearer to say that the value is a *list* that
> is *used* as a stack.
> 
> 3. Better to just say "vectors", not "sets", since the order is
> important (and the reader is right-away presented with the vector
> description).

And I forgot perhaps the most important of all: please describe each
of the vector elements.  Say what they correspond to, e.g.,
`isearch--state-barrier' corresponds to variable `isearch-barrier'
(which has a doc string describing it).