Clear the four -Wdocumentation warnings

[karpovantonme]

Building the two aggregate headers with -Wdocumentation gives four warnings. This clears them.

$ clang++ -Wdocumentation -fsyntax-only -I include -std=c++14 agg.cpp
special_functions/lambert_w.hpp:781:14: warning: parameter '-term' not found in the function declaration
distributions/find_scale.hpp:63:7: warning: not a Doxygen trailing comment
distributions/find_scale.hpp:64:7: warning: not a Doxygen trailing comment
distributions/hyperexponential.hpp:57:11: warning: not a Doxygen trailing comment

Two different things behind them.

One real documentation mismatch

lambert_w.hpp:781

//! \param _z Lambert W argument z.
//! \param -term  -pow<18>(z) / 6402373705728000uLL
//! \param _k number of terms == initially 18

lambert_w0_small_z_series_term(T _z, T _term, int _k)

A hyphen where the other two have an underscore. Doxygen looks for a parameter named -term, does not find it, and _term ends up undocumented.

Three accidental Doxygen markers

//< and /*< are how Doxygen marks a trailing comment, and in these three places the sequence turns up by accident.

In find_scale.hpp it is inside commented-out debug output, where a line that used to begin with << quantile(...) became //<< quantile(...):

//cout << "z " << z << ", p " << p << ",  quantile(Dist(), p) "
//<< quantile(Dist(), p) << ", z - mean " << z - location

In hyperexponential.hpp it is the note on the unnamed namespace, namespace /*<unnamed>*/ {.

Neither is a documentation error, just a character sequence that reads as one. A space fixes both and the comments say the same thing.

I checked through boost/math/special_functions.hpp and boost/math/distributions.hpp, so this covers what those two pull in rather than every header in the library. -fsyntax-only stays clean after the change.

[NAThompson]

@karpovantonme : Rebased and am cycling CI. Should be merged shortly . . .