Various improvements; joint work with beck@:

Various improvements; joint work with beck@:
1. Explain up front what "ASN1_TIME" is (suggested by beck@, wording by me).
2. For opaque structs, use the generic term "object",
like we already do it in many other LibreSSL manual pages.
3. Drop some redundant phrases.
4. Improve the EXAMPLES section (by beck@, with fixes by me).
6. Add a STANDARDS section.
...and some other minor polishing.
OK beck@