doc/strace.1: elaborate on support of multiple PIDs as a -p argument

While it was mentioned that -p "`pidof PROG`" syntax is supported,
it was not explicitly stated what characters can be used as separators
(like newline, which is used by pgrep, as noted in process_opt_p_list).

* doc/strace.1.in (.SS Startup): Explicitly mention that multiple PIDs
can be provided as an argument to a single -p option;  list all
supported separator characters;  mention possible -p "$(pgrep PROG)"
syntax.

doc/strace.1.in

@@ -323,8 +323,15 @@ options can be used to attach to many processes in addition to
 (which is optional if at least one
 .B \-p
 option is given).
+Multiple process IDs, separated by either
+comma (\(lq,\(rq), space (\(lq \(rq), tab, or newline character,
+can be provided as an argument to a single
 .B \-p
-"`pidof PROG`" syntax is supported.
+option, so, for example,
+.B \-p
+"$(pidof PROG)" and
+.B \-p
+"$(pgrep PROG)" syntaxes are supported.
 .TP
 .BI "\-u " username
 .TQ