DOC: Tweak the 1.8.0 release notes.

Add the new nan functions and linear algebra gufuncs to highlights.
Tweak some rst formatting.
Merge pull request #3870 from charris/backport-1.8-3869

backport-1.8-3869
BUG: Include sunmath.h in npy_math.h when __SUNPRO_CC defined.

The include is needed for isinf.

Closes #3871.
Merge pull request #3874 from charris/backport-1.8-3872

Backport 1.8 3872