Clarified importance of sticking to a single host Lisp.

git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1287 5d84a466-a78b-0930-a311-59b22171a541
Quoting a comment from (defxdoc note-6-5 ...) in
/projects/acl2/devel/books/system/doc/acl2-doc.lisp:

; Enhanced error message from Step 1 of certify-book to clarify how packages
; may be the issue, pointing to :DOC certify-book.  Thanks to Ben Selfridge for
; bringing this issue to our attention and for helpful follow-up emails.

Updated :doc certify-book accordingly.

[Note: I accidentally checked in much of this change in acl2-devel svn
revision 1287, i.e., the preceding check-in.  But I modified it here
to shorten the new error message from certify-book, instead expanding
:doc certify-book.]


git-svn-id: http://acl2-devel.googlecode.com/svn/trunk@1288 5d84a466-a78b-0930-a311-59b22171a541