proc_pid_fdinfo.5: Reduce indent for most of the page

proc_pid_fdinfo.5: Reduce indent for most of the page

When /proc/pid/fdinfo was part of proc.5 man page the indentation made
sense.  As a standalone man page the indentation doesn't need to be so
far over to the right.  Remove the initial tagged pragraph, move the
"since Linux 2.6.22" to a new HISTORY subsection.

Suggested-by: "G. Branden Robinson" <g.branden.robinson@gmail.com>
Message-Id: <20241206073828.1119464-1-irogers@google.com>
[alx: ffix]
Signed-off-by: Alejandro Colomar <alx@kernel.org>