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

[rhdunn]

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.