[trivial] Updates the ADD_SERIALIZE_METHODS documentation #14015

This pull request fixes a small error in the ADD_SERIALIZE_METHODS documentation.

At the moment the ADD_SERIALIZE_METHODS macro adds two methods to a class:

template<typename Stream>void Serialize(Stream& s) const { ... }
template<typename Stream>void Unserialize(Stream& s) { ... }

ADD_SERIALIZE_METHODS used to add three methods to a class:

size_t GetSerializeSize(int nType, int nVersion) const {...}
template<typename Stream>void Serialize(Stream& s, int nType, int nVersion) const  {...}
template<typename Stream>void Unserialize(Stream& s, int nType, int nVersion)  {...}

size_t GetSerializeSize(int nType, int nVersion) const was removed from the ADD_SERIALIZE_METHODS macro in 657e05ab2e87ff725723fe8a375fc3f8aad02126, but the documentation was not updated accordingly at the time.

This pull request corrects this mistake.

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #10785 (Serialization improvements by sipa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Unless there is somebody who thinks this PR is useful, I will close it shortly, because the ADD_SERIALIZE_METHODS documentation has been updated adequately in #10785.