Which CentOS 7 package provides the "boot" manpage?

The boot(7) manpage is provided by the man-pages project. In CentOS, this is packaged as man-pages, but a few man pages which are considered irrelevant for CentOS are excluded, including boot(7). boot(7) is considered irrelevant because it describes the System V-style boot process (using inittab and boot scripts).

This does mean that CentOS (and RHEL, and Fedora) should patch out the reference to the man page...

That manual was removed from the CentOS man-pages package in 2014 when CentOS switched to using systemd. From the changelog at https://centos.pkgs.org/7/centos-x86_64/man-pages-3.53-5.el7.noarch.rpm.html:

2014-02-11 - Peter Schiffer  - 3.53-5
- resolves: #1058101
added note about default values to the nscd.conf(5) man page
- resolves: #1059829
added three pthread_mutex* man pages from POSIX.1-2008
- resolves: #1062906
removed boot(7) man page

The resolved bug number references Red Hat’s bug tracker but the bug isn’t public.