Remove some KSE references from ps(1).

• Simplify the description of -H to assume 1:1 threading.
• Drop 'process' from description of 'lwp' field and the corresponding XO field name.
• Do add an expansion of LWP in the description of 'lwp' and 'nlwps'.
• Add 'tid' as an alias for the 'lwp' field.

Reviewed by: imp, kib (older version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14021