doc: Mention vec=vec.head(n) in aliasing page.

--HG--
extra : transplant_source : %00%CE%14%B1T%F1%93%AC%83j2%1C%16%06%C7%8A%88%C2%5D%A5
Fix some doxygen errors and warnings.

--HG--
extra : transplant_source : %F3%06%E9%1BeJ%F3z%85OBX%D6%9B%CCumT%27S