chattr.1: improve attribute formatting with labels and indented paragraphs

chattr.1: improve attribute formatting with labels and indented paragraphs

By convention, lists of options in man pages use a label followed by an
indented description, such as this example from the Options section:

     -R     Recursively change attributes of directories and
            their contents.

But the Attributes section places the available attributes mid-sentence,
which makes it visually more difficult to parse:

     A file with the 'a' attribute set can only be opened
     in append mode for writing.  [...]

     When a file with the 'A' attribute set is accessed, its
     atime record is not modified.  [...]

This patch places a label beside each attribute description, which (in
my opinion) improves readability, especially when visually skimming the
list.  For example:

     a      A file with the 'a' attribute set can only be
            opened in append mode for writing.

     A      When a file with the 'A' attribute set is accessed,
            its atime record is not modified.

Signed-off-by: Jeremy Visser <jeremyvisser@google.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>