zfsprops.7: Add note about comma-separation

[debdrup]

This change primarily seeks to make implicit documentation explicit, as
it is not outright stated that options should be comma-separated, nor is
there a reason given for it.

Motivation and Context

Having implicit documentation can lead to important details being lost,
and makes it harder to point to anything specific.

Description

An extra paragraph has been added to make it clear that the options are
comma-separated, and that this is different from what's expected in
exports(5). Additionally, the reason for the choice is expanded upon,
and although I wasn't able to find a commit to back this up, I think it
might be nice to have something to refer to in the commit message.

How Has This Been Tested?

It has been tested using mandoc on FreeBSD 13, rendered to both less(1)
and as a HTML using -Thtml

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style
  requirements.
• I have updated the documentation accordingly.
• I have read the contributing
  document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain
  Signed-off-by.

[ericloewe]

Don't you have a duplicate "with" in there?

[behlendorf]

Aside from the typo this looks fine. The comma separated format originally comes from what was used on Solaris, which I presume was done to keep the parsing simple.