MONGOID-5590 - Docs: Clarify that shard key dot limitation comes from MongoDB server

[johnnyshields]

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As per DOCS-15993, the limitation of dot in shard keys comes from MongoDB server, not from Mongoid.

This is a follow-up to https://github.com/mongodb/mongoid/pull/5588

[johnnyshields]

@jamis ready to merge.

[johnnyshields]

@jamis any reason why this doc-only PR can't be merged?

[jamis]

@johnnyshields -- I think this might not be true anymore. For v6 of the database, I think you're right (https://www.mongodb.com/docs/v6.0/core/dot-dollar-considerations/), but the current docs (https://www.mongodb.com/docs/manual/core/dot-dollar-considerations) make no mention of that restriction. The page you're linking to in the docs seems to flatly contradict the statement that dots and dollar signs are not supported.

If you can find another page for the current server version (v7) that says that dots and dollar signs are forbidden in shard keys at the server level, please do update the PR. Otherwise, I think this PR might be obsolete.