Feature/issue 1025 change function documentation

#### Summary:

Make the function documentation more consistent. Moved detailed function doc to the C++ documentation. Wrote mathematical definition of function in the manual.
* Note: I want to make another pass, but this will be in good enough of a place for Bob to break out the manual into pieces.

#### Intended Effect:

- Moves the detailed function level documentation to developer doc in the form of doxygen documentation.
- Reduces the doc of functions in the stan-reference to more math-like definitions.
- Minor rewording of these functions for consistency.

#### How to Verify:

- Build the manual and inspect.
- Build the doxygen and inspect.

#### Side Effects:

Added `\fitemnobody` and `\fitemindexnobody` macros to stan-manuals.sty for fitems without a body paragraph.

#### Documentation:

Yes. This is all doc.

#### Reviewer Suggestions:

@bob-carpenter