Clarify undefined-behavior entry for concurrent use of teams

[BryantLam]

This PR clarifies the undefined-behavior "Concurrent use of teams" entry that was a discussion point on the April 27, 2020 v1.5rc2 meeting.

• Changed focus of first sentence to warn that team-based collectives are not thread-safe if operating on the same team object. All team-based collectives take a \VAR{team} argument input.

• Amended second sentence to now say "may be undefined behavior". This precludes concurrent thread-safe usage of a team in operations such as shmem_team_translate_pe.

• Explicitly emphasized the example as concurrent collective on the same team object, SHMEM_TEAM_WORLD. This leads the application developer toward a viable solution for this usage: clone the world team (though it does beg the question of whether implementers could also clone the world team to deconflict from application usage of SHMEM_TEAM_WORLD).

[BryantLam]

All: Feel free to help review this text. I only tagged the reviewers that had talking points on the call or in the past.