[patch] Doxygen fixes for std::string and std::num_get

[Paolo Carlini]

Hi Jonathan and sorry for the delay

>OK for mainline, and 4.0 when it thaws?
>  
>
It seems definitely ok to me and nobody objected 'til now, therefore,
definitely go ahead, mainline and, asap, 4_0-branch.

Thanks!
Paolo.