Fix some warnings in Kigo

    Fix doxygen warnings for Stone constructors in games/stone.h

"float value" parameters of Stone constructors had no description

Fix it by adding a corresponding description: it is an estimated value
for a potential next move. It is used when the user is requesting
a hint.

    Make single parameter constructors explicit

krazy2 complained:

    Check for C++ ctors that should be declared 'explicit' [explicit]...
    OOPS! 2 issues found!

    src/game/score.h: line# 45 (1)
    src/gui/config/generalconfig.h: line# 45 (1)

Fix it by making them explicit.

REVIEW: 120076