DDoc: elaborated on 'scalar type' and 'basic type'

Previously it wasn't completely obvious what a scalar or basic type is in std.traits; now it's described explicitly.
Update comments to match new std.regex examples

Example was switched to matchAll and 'g' was removed but the comment got left in. Another comment about match wasn't switched to matchAll.
Merge pull request #1867 from eco/patch-13

[minor] Remove comment about 'g' flag (no longer applies)
Merge pull request #1868 from kiith-sa/patch-8

DDoc: elaborated on 'scalar type' and 'basic type'