Add notice that forcefully unmount is not supported on Linux.

[oshogbo]

#9700 # Motivation and Context
Add notice that forcefully unmount is not supported on Linux.
As discussed: #9904 (comment)

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)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux 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.

[codecov]

Codecov Report

Merging #10013 into master will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #10013    +/-   ##
========================================
+ Coverage      79%      79%   +<1%     
========================================
  Files         386      386            
  Lines      122431   122431            
========================================
+ Hits        97023    97332   +309     
+ Misses      25408    25099   -309

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <ø> (+1%)	⬆️

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