man: extend documentation about TPM2 PCRs

man: extend documentation about TPM2 PCRs

This is an effort to compile a somewhat complete list how PCRs are
actually used on Linux systems these days. It contains data from: the
UEFI PC spec, the shim, the IMA, grub documentation.

I validated these PCRs to some level in the sources.

The grub specific stuff I only added in comments, since I was too lazy
too validate it (also, meh, grub).

It also gives people a hint on which PCR to bind to (and maybe kind of
an explanation of our default choice).