xfrm: Update documentation

xfrm: Update documentation

The ip(8) man page and the "ip xfrm [ XFRM-OBJECT ] help" command output
are updated to include missing options, fix errors, and improve grammar.
There are no functional changes made.

The documentation for the ip command has many different meanings for the
same formatting symbols (which really needs to be fixed). This patch makes
consistent use of brackets [ ] to indicate optional parameters, pipes | to
mean "OR", braces { } to group things together, and dashes - instead of
underscores _ inside of parameter names. The parameters are listed in the
order in which they are parsed in the source code.

There are several parameters and options that are still not mentioned or
need to be described more thoroughly in the "COMMAND SYNTAX" section of
the ip(8) man page. I would appreciate help from the developers with this.

Signed-off-by: David Ward <david.ward@ll.mit.edu>