Updated zfs_dbgmsg_enable documentation to be more accurate.

[rincebrain]

Motivation and Context

#11984

Description

Changed the default specified for zfs_dbgmsg_enable, added clarification of interaction with zfs_flags.

(If people think that instead the default should be changed, that can be done too.)

How Has This Been Tested?

man man/man5/zfs-module-parameters.5 didn't produce any errors, make checkstyle didn't either.

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.

[ghost]

You could bump the date on the man page, too.