DOC: config: Fix documentation about dot character in variable names

DOC: config: Fix documentation about dot character in variable names

In the section 2.8 (Variables), the doc character ('.') was not listed in
the set of characters allowed in a variable name. However it is an
error. The dot character is allowed. It cannot be used in the scope. But it
is allowed in the name.

The bug was introduced with the commit b643fbb1a ("DOC: config: Add a
dedicated section about variables"), when the section 2.8 was added.

This patch could be backported as far as 2.8.