Miscellaneous small :doc and I/O fixes.

Added :doc update-nth-array (fixes broken link).

Updated :doc relnotes for books/projects/hybrid-systems/.

Eliminated a bogus warning at startup about gc-verbose not being
supported, which occurred in hons-enabled ACL2 for other than lisps
where gc-verbose is supported, namely (as of this writing) CCL, CMUCL,
and GCL.