#510 Restrictions for counter names not documented

[Michael Schlenker]

The counter names are not arbitrary strings, strings
containing namespace qualifiers are simply not working:

(bin) 69 % counter::init ::foo -hist 20
can't access "counter::T-::foo": parent namespace
doesn't exist

Possible solutions:
a) document this limitation
b) rework most of the internals, probably slowing down
counter

Just documenting it might be enough.