BOOST_TEST_GLOBAL_FIXTURE documentation misleading

[Mondlied]

The documentation of BOOST_TEST_GLOBAL_FIXTURE makes no mention of a restriction of the class names that can be passed. Passing a class name including the namespace leads to a compiler error though. The documentation should be adjusted to indicate this (or the macro adjusted, assuming this is even possible.)

Documentation document: https://github.com/boostorg/test/blob/develop/doc/test_organization/fixtures.qbk (Section "Global fixture")

Example of the issue

namespace foo {
    struct Bar {
    }
}

BOOST_TEST_GLOBAL_FIXTURE(foo::Bar);

It's pretty simple to work around this issue, but the documentation should mention the restriction on the macro parameters.

Workaround for the above case:

namespace foo {
     BOOST_TEST_GLOBAL_FIXTURE(Bar);
}

Alternatively a typedef/using could be used, but this could lead to name conflicts that are avoided by putting BOOST_TEST_GLOBAL_FIXTURE inside the namespace.

[raffienficiaud]

Merged in #337 rev b52b0ea.