Add a sentence about full ext4 read/write support the filesystems chapter in the handbook

[bcr]

Summary

This is referencing PR 234979 adding a sentence that ext4 has full read and write support as of FreeBSD 12.0-RELEASE.
@pfg: Can you confirm that it has full write support as the man page suggests? If you have further textual suggestions for that part of the handbook, let me know. Thanks!

Test Plan

1. Apply the patch to the doc root
2. Build the handbook by running make
3. Check the resulting filesystems.html

[pfg]

Yes, we do fully support ext4 read and write in FreeBSD 12+. We also support Extended Attributes, ACLs, and most features with the exception of journalling and encryption. As mentioned in BSDNow, 12.1 will have a DTrace provider.

Previous FreeBSD versions can access ext4 in read/write mode through the sysutils/fusefs-ext2 port.

[pfg]

Adding fsu@ in case he wants to mention something else :).

[bcr]

Update patch with more information about capabilities and what package people can use on prior FreeBSD versions. Thanks @pfg.
While in the neighborhood, fix an issue that textproc/igor reported in that <note> section.

[fsu]

I am not sure about "filesystem has full read and write support for ext4" because there some ext4 features, that uses rarely and we does not support it.
But may be do not dig into the details in this place.