Allow operator/ and operator/= documentation to be merged.

This is a fix for documenting `operator/` and `operator/=` in markdown. For example:

    struct vector {
        const vector &operator/=(int scalar);
    };

    vector operator/(const vector &v, int scalar);

The XML/tree construction is working for this case. However, given the markdown:

    #<cldoc:vector::operator/=>

    Description.

    #<cldoc:operator/>

    Description.

The program fails with e.g. `Unknown type '=' for id 'operator'`. This patch prevents that issue.