fix: update documentation of custom _id overriding in discriminators

[sam-mfb]

Summary

This fixes Issue 8588 which appears to be an artifact in the documentation from before this fix to how the discriminator code handled a custom _id in the base schema. As I understand it, in the old code the implicit discriminator schema would still override the base, unless _id was set to 'false' in the discriminator options. This is what the old documentation still reflects (at least in part). However, it appears that now a custom base _id always overrides the discriminator _id. This updates the documentation to reflect that behavior.

Examples

This is a test file. The tests still run correctly--it was just the descriptions that didn't match.

[vkarpov15]

Thanks!