Reimplement: Fix doc warnings

This issue tracks reimplementation of the work from stale PR #2305, which is being closed because it is too out of date to merge directly.

    Original PR: https://github.com/clockworklabs/SpacetimeDB/pull/2305
    Original author: @mamcx
    Original branch: `mamcx/fix-docs-comments`
    Base branch: `master`

    ## Original PR summary

    # Description of Changes

Fix as many warnings caused by cargo doc.

Some that stay:

• Of the form warning: public documentation for X links to private item Y
• The top doc on crates/core/src/subscription/subscription.rs This generate unresolved link warnings but looks to me that need a full rewrite c.c. @joshua-spacetime.

Expected complexity level and risk

0

Testing

• Run cargo doc --all. Because we have a naming collision:

Error: document output filename collision
The lib `spacetimedb` in package `spacetimedb-core v1.0.0-rc4 (.../space/SpacetimeDB/crates/core)` has the same name as the lib `spacetimedb` in package `spacetimedb v1.0.0-rc4 (../space/SpacetimeDB/crates/bindings)`.
Only one may be documented at once since they output to the same path.

... then I switch adding doc =false in each to let me see all the warnings.

    ## Follow-up

    - Reimplement this change in a fresh PR against current `master`.
    - Carry forward any still-relevant context from the original PR discussion and review.
    - Link the new implementation PR back to the original stale PR for historical context.

Start by reviewing stale PR #2305 and its discussion, then run cargo doc --all while checking the crates/core and crates/bindings documentation setup. Review crates/core/src/subscription/subscription.rs and preserve the warnings explicitly identified as requiring a rewrite; done means the remaining actionable documentation warnings are fixed and the documented exceptions are understood.