added NaN test for acosh
updated nan test for acosh
updated agrad/fwd/acosh
added NaN tests for agrad/fwd/acosh
fixed NaN bug in agrad/rev/acosh and switched to no namespace version of acosh
updated bound tests for acosh
added NaN and derivative doc for acosh in manual
updated fwd/matrix tests
Merge branch 'develop' into feature/test-nan-acosh

Conflicts:
	src/docs/stan-reference/appendices.tex
Merge branch 'develop' into feature/test-nan-acosh

Conflicts:
	doc/stan-reference-2.4.0.pdf
	src/docs/stan-reference/appendices.tex
Merge pull request #948 from stan-dev/feature/test-nan-acosh

Feature/test nan acosh