docs: list and describe all sysfs files

[kdave]

Add section to section 5 manual page (btrfs-man5.asciidoc) with description of all files exported in /sys/fs/btrfs. Some of them are already there, eg. feature bit description, so these can be only referenced. For each sysfs file write: version when introduced, format of data and human readable meaning.

[kdave]

Sysfs docs are in ch-sysfs.rst, added in 6.1.2. The value format is not there, in most cases it's just a number. Compound files like error_stats or commit_stats should be documented though.