Document spacewalk-data-fsck (troubleshooting)

[keichwa]

spacewalk-data-fsck is a debugging and troubleshooting tool. It helps after fatal crashes when inconsistencies are detected. You do not need it to remove packages from disk after using of spacewalk-remove-channel.

I would move this into the troubleshooting section.

Originally posted by @mcalmer in #3631 (comment)

[keichwa]

Proposal for inspiration:

===  Remove the Packages from the Filesystem

.Procedure: Removing the Packages from the Filesystem
. The following command checks for anything that still exists in the filesystem, but not in the {productname} database, and removes it.
  On the command line of the container host, run the following command:
+
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mgrctl exec -ti -- spacewalk-data-fsck -r -S -C -O
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[NOTE]
====
The final cleanup command can take several minutes, or even hours to complete.
====