docs: clarify wipefs --force description for partition-table signatures

docs: clarify wipefs --force description for partition-table signatures

The current description states that --force "is required in order to erase
a partition-table signature on a block device", which is misleading.
According to the source code (do_wipe() in wipefs.c), --force is needed
for mounted filesystems and for nested partition tables on non-whole-disk
devices, not for all partition tables on all block devices.

Update the text to say "a nested partition-table signature on a non-whole
disk device", consistent with the note earlier in the man page.

Fixes #2447