[mono-api-html] Prettier output using singular and plural forms in sections
[mono-api-html] Allow a lax processing mode of the XML files

This is useful when the files needs to be pre-processed (e.g. a major change between the assemblies internals) and where some duplicate entries might be created (e.g. merged types/namespaces).

Note: normally the mono-api* tools should not produce such files (so it's better not used unless required)