Enable comments after field Type.name

[rbalicki2]

e.g.

field Foo.bar
"""
asdfasdfsadfas
"""
{
  baz
}

Is it ugly? Maybe. But it works with iso.ts, and people will get over it.

There are several steps here:

• first, move the parse_optional_description line later
• second, do not return an error if the description is present
• third, remove the now-unused error