MFC r326748:

Document the existence and precision of the remaining long double
functions for which an imprecise stub implementation was added in
r255294, namely powl(3) and tgammal(3).

Submitted by: Steve Kargl

MFC r326753:

Correct r326748, indicating that tgammal(3) is mapped to tgamma(3), not
to itself.

Noticed by: jilles