tput(1): Several enhancements for the manual page

• a couple of descriptions are incomplete
• synopsis doesn't show that all arguments are optional
• missing an ENVIRONMENT section with TERM mentioned

PR: 84670
Submitted by: Gary W. Swearingen <garys at opusnet dot com>
Reviewed by: bcr
Approved by: bcr
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26009