Add information on how to load GEOM_MIRROR into the kernel at boot time and while the system is running to the SYNOPSIS section of gmirror.8

[bcr]

Summary

This patch was triggered by PR 234997. It turns out that the other GEOM module man pages also don't have this information (they were probably written at the same time or were using the same template). When this review is done and committed, we should also do the same for the other GEOM modules in head/lib/geom/*/<module>.

Checked with mandoc -Tlint and textproc/igor (only my changes).

Test Plan

a) Apply patch to FreeBSD head
b) View man page: man ./head/lib/geom/mirror/gmirror.8

[allanjude]

I might reverse the order of these, since loading the module is the recommended action, and compiling it into your kernel is for people who want to do custom kernels.

Not sure if we also want to be explicit that loader.conf lives in /boot

[bcr]

I copied the header from existing man pages for consistency. If we want to switch the module loading to the beginning (which I think is a good idea), we would have to (in that order):
a) change it in this man page
b) add it to all the other geom modules man pages for consistency and
c) do the same for the rest of the man pages we have that support loading it as a module.

a) and b) are fairly easy, whereas c) is a bigger effort to find and change all those pages.