Document buffer-local mappings

[olmokramer]

It took me a while until I realized that it is possible to achieve buffer-local mappings with MapExpress and MapSubpress, by prepending the lhs of the mapping with <buffer>, e.g. MapExpress <buffer>cd '/* ' . v:val . ' */'. I tried it with a space between the <buffer> and the lhs, but that did not work. Since some of the mappings only really make sense in certain filetypes I thought it would be nice if this little fact were documented.