Given

  (require 'cl-macs)
  (cl-defstruct foobar
    (baz nil :documentation "an important piece of information"))

Now, (describe-symbol 'foobar-baz) will include "an important piece of
information".

However, (describe-symbol 'foobar) will not:

Name Type Default
———— ———— ———————
baz t nil

... even though the intention to include it can be seen by looking for
the string ':documentation' in `cl--describe-class-slots`.

This is because `cl--describe-class-slots` accidentally uses
`alist-get` instead of `plist-get`.

With the attached patch, (describe-symbol 'foobar) works as intended:

Name Type Default Doc
———— ———— ——————— ———
baz t nil an important piece of information

Note: `cl--describe-class-slot' (a different function) appears to have
the same bug, but I have no idea how one would go about adding class
slots to a struct and therefore no way of testing any changes to this
other function.

Could it be a code path that is never executed anyway?  I decided to
leave it alone, this patch should be an improvement regardless.