strace.1.in: describe time format specification

strace.1.in: describe time format specification

Commits v5.3~74 and v5.3~73 have introduced an extended syntax
for time interval sizes specification, but the relevant descriotion
was lacking.  Fix it by adding the relevant section to the man page
and reference to it in the descriptions of the respective options.

* strace.1.in (.SH OPTIONS): Rewrite descriptions of -O,
-e inject=delay_enter, and -e inject=delay_exit values, refer to section
"Time specification format description".
(.SS "Time specification format description"): New section.

Complements: v5.3~74 "delay: use parse_ts for parsing delay value"
Complements: v5.3~73 "count: use parse_ts for parsing overhead value"