revised doc, section titles, and added examples for sort functions
revised doc, section titles, and added examples for sort functions

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan