please use before-save-hook instead of write-file-hooks

[Paul Eggert]

The write-file-hooks feature of Emacs has been declared obsolescent 
since Emacs 22.1 (2007), so it's time to use this feature's suggested 
replacement in the Texinfo files that use the feature. For more details, 
please see this thread:

https://lists.gnu.org/r/bug-gnulib/2018-03/msg00008.html

Proposed patch attached. This patch doesn't affect other files in the 
Texinfo source-code repository that use the obsolescent feature, as 
these files are downstream from other packages that have already 
upgraded and so should be fixed automatically on the next autoupdate.

texinfo.diff
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