docs: restrict the set of characters for info keys

docs: restrict the set of characters for info keys

When parsing the top level comment of a file, apibuild.py used
to split on any ':' character of a line regarding the first part
as a key for a setting, e.g. "Summary". The second part would then
be assigned as the value for that key.

This means you could not use a ':' character inside those comments
without ill effects.

Now, a key must consist solely of alphanumeric characters, '_' or '.'.