btrfs-progs: doc: fix argument notation and typos

All user-supplied values should be enclosed in <...> to distinguish
them from verbatim strings.

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: doc: remove text for unmerged features

The asciidoc conversion was done on a development branch and there are
portions of text that do not reflect the code.

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: doc: autoformat user-supplied arguments by sed

Convert all values enclosed between <...> to italic before it goes to
asciidoc processor.

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: doc: make all commands and subcommands bold

Italic format is used for parameters and values.  also this makes the
text visually separated,

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: doc: update the Availability section

Does not reflect the current state. The wiki contains more details on
the first page.

Signed-off-by: David Sterba <dsterba@suse.cz>